The county seat is a key area for coordinated promotion of new-type urbanization and rural revitalization, and has an important strategic position in the Chinese-style modernization. This paper takes Taihang Mountain Area of Hebei Province as an example, analyzes the coupling coordination relationship and spatio-temporal evolution of the three sub-systems of population, land and municipal infrastructure in the process of county urbanization from 2004 to 2021, and discusses the development mechanism of “population-land-municipal infrastructure” coupling coordination to promote the county urbanization. The population attraction of the county seats in Taihang Mountain area of Hebei Province continues to increase, the construction area of the county seats show an expanding trend, but the expansion speed shows a declining trend, and the municipal infrastructure are steadily upgraded. From the perspective of two-subsystems, the coupling coordination degree of “population-land” decreased from 0.5626 to 0.5112, and was in a state of barely coordination; the coupling coordination degree of “land-municipal infrastructure” increased from 0.4995 to 0.5084, and the coupling coordination degree of “population- municipal infrastructure” was in a state of precariousness but relatively stable. From 2004 to 2021, the coupling degree of "population-land-municipal facilities" in the county seats of Taihang Mountain area of Hebei Province increased from 0.8372 to 0.8835, showing a benign coupling trend, and the coupling coordination degree was moderate. The spatial characteristic shows that the north and south counties are relatively coordinated, but the middle counties are on the verge of subordinate. From the perspective of lag type, population urbanization lags behind land urbanization, and the level of county municipal infrastructure lags behind population urbanization and land urbanization. To improve the urbanization quality, the county seats in Taihang Mountain Area of Hebei Province should start from improving the coordination level of supply and demand of “population-municipal infrastructure” and perfecting the spatial matching mechanism of “land- municipal infrastructure”.