The thermal environment in cold urban areas during transitional seasons significantly impacts residents’ outdoor activities and overall quality of life. In high-latitude cities like Shenyang, harsh winters dominate the annual calendar, while transitional seasons are short and characterized by low temperatures, strong winds, and limited solar radiation. These conditions create suboptimal microclimatic environments that restrict outdoor activities and reduce the appeal of public spaces. Improving the thermal environment during these periods is therefore critical for extending residents’ outdoor activity durations, promoting healthier lifestyles, and enhancing urban livability. Creating a suitable microclimate and promoting residents’ exposure to the outdoor environment has become an important goal of neighborhood environmental optimization. This study addresses this challenge by investigating the role of blue-green spaces (BGES) in mitigating thermal discomfort in cold urban neighborhoods, using Shenyang City as a case study. Blue-green spaces, defined as the integration of vegetation (green spaces) and water bodies (blue spaces), are essential components of urban ecosystems. By influencing microclimatic conditions through processes such as evapotranspiration, shading, and heat absorption, BGES can significantly improve thermal comfort in urban areas. However, existing research on BGES and thermal environments has predominantly focused on temperate or tropical regions, with limited attention to cold urban contexts. Furthermore, the complex interactions between BGES characteristics (e. g., spatial configuration, vegetation types, water body sizes) and thermal dynamics in cold regions remain underexplored. This study fills this gap by examining the impacts of BGES on thermal environment during transitional seasons in Shenyang, a representative cold city in Northeast China.The methodology employed in this study involves several key steps. Firstly, seasonal differentiation was conducted using historical meteorological data from the National Meteorological Science Data Center. Transitional seasons were defined as March 25 to April 8 and October 12 to November 6, totaling 40 days. Secondly, spatial scale analysis was performed across multiple spatial scales (300 m to 1 000 m radii) to determine the optimal scale for investigating BGES impacts on the thermal environment. This step is crucial because the effectiveness of BGES in regulating thermal conditions can vary significantly depending on the spatial context. Thirdly, eight BGES indicators were selected, categorized into capacity, structure, and morphology. These indicators were chosen based on their relevance to thermal regulation processes and their measurability using remote sensing. Fourthly, a comprehensive thermal environment evaluation index was developed using six meteorological parameters: surface temperature, wind speed, humidity, minimum temperature, average temperature, and cold intensity. The weights of these parameters were determined using the AHP-E. Finally, BRT modeling was employed to analyze the nonlinear relationships between BGES indicators and thermal comfort. The results of this study reveal several important findings. Firstly, a 1 km2 block scale was identified as the optimal spatial unit for studying the impacts of BGES on the thermal environment in Shenyang during transitional seasons. At this scale, BGES indicators exhibit the strongest influence on thermal environment. Secondly, among theBGES indicators examined, TSR, GCR, and WAR were found to be the most significant contributors to thermal comfort, with relative contributions of 30.79%, 16.37%, and 15.15%, respectively. Thirdly, marginal effect analysis revealed threshold values for these indicators, providing practical guidance for urban design. For instance, TSR should be controlled within a 5:1 ratio to balance vegetation competition and airflow, GCR should be maintained between 30% and 48% to avoid land resource waste while ensuring sufficient cooling effects, and WAR should be kept between 5% and 10% to optimize humidity and temperature regulation. The implications of these findings are multifaceted. For urban planners and designers, the study underscores the importance of integrating BGES into a connected ecosystem to enhance scale and connectivity, strategically increasing BGES in land-scarce areas, and optimizing the morphological complexity of BGES to maximize thermal regulation benefits. Furthermore, the identified thresholds provide actionable targets for design interventions, ensuring that limited resources are allocated effectively. From a research perspective, this study advances the understanding of BGES-thermal environment relationships in cold regions, offering a robust methodological framework for future studies. The combination of BRT modeling, multi-scale analysis, and comprehensive thermal evaluation provides a replicable approach for investigating similar issues in other climatic contexts. In conclusion, this study demonstrates that BGES can play a transformative role in improving thermal comfort in cold urban areas during transitional seasons. By strategically designing and managing BGES, cities like Shenyang can create more livable, resilient, and climate-adaptive environments that enhance residents’ well-being and encourage outdoor activities. The insights gained from this research not only contribute to the scientific literature but also offer valuable references for policymakers, planners, and practitioners working toward sustainable urban development in cold regions.