With the global expansion of industrial civilization, nature is neglected and destroyed unprecedentedly. The issue of intergrowth with nature becomes more urgent than before. Alvar Aalto, a famous Finland modern architect who rooted architecture design in Finland’s nature and culture, explored a new way of modern regional architecture which is harmonious with nature and achieved great success. The rational study on the nature theme in Aalto’s regional design is of great significance for contemporary development and architecture design. After comparing the original documents, this paper firstly carries out an in-depth study on the landscape and cultural origin of nature theme in Aalto’s regional architecture design. Secondly, the approaches to express nature theme in Aalto’s regional design are discussed and analyzed in detail. Then, it is found out that the love for amazing beauty of Finland’s nature and the cherishing of Finland’s cultural value are the important bases for Aalto’s successful presentation of nature theme in regional architecture design, and the innovative use of paradigm of Finland vernacular court-yards and multiple metaphor use of Finland landscape are two key methods for Aalto’s impressive expression of nature theme.