Abstract: Under the background of vigorously developing small and medium-sized cities and promoting balanced urbanization, an accurate analysis of the difference of urban green construction in county-level cities will contribute to the exploration of the problems and countermeasures of urban green development. This paper takes the municipal administrative areas as the sample unit, uses green rate of built district to characterize urban green construction, adopts spatial analysis and mathematical method to reveal the spatio-temporal variation characteristics of green rate of built district of county-level cities from 1996 to 2015. The result shows: 1) Compared with other administrative cities, county-level cities are the lowest level from early stage to the present. The green rate of built district rises steadily, but the growth rate decreases gradually. 2) The relative difference between county-level cities decreases rapidly, but the absolute difference decreases slowly and remains at a high level. 3)Green rate of built district presents the spatial distribution pattern of “East > Central> West > Northeast”, and the spatial variation is relatively stable. The central part is growing faster, while the northeast is backward. This paper clarifies the spatio-temporal variation characteristics of green rate of built district of county-level cities in China in the past 20 years, to provide a theoretical basis for the urban green space construction of county-level cities.