Rapid development and application of urban space-time big data have provided a new data environment and technical means for identifying urban functional areas. However, the literature regarding mixed urban functional areas detection in the field of urban public service facilities is still lacking. Using spatial point-level data of nine categories of urban public service facilities in Beijing with consideration of their rank and quality, this paper employed cumulative opportunity method to measure urban public service facilities' accessibility in Beijing at 1 km×1 km grid scale, and further emphatically analyzed the mixed functions and influencing factors of urban public service facilities. The results show that the spatial distribution of urban public service facilities' cumulative opportunity accessibility in Beijing presented a similar characteristic of central agglomeration. While the specific spatial patterns and coverage areas of urban public service facilities varied by their categories. In addition, functional areas of urban public service facilities in Beijing were divided into the five types: single functional areas, mixed functional areas with single facility oriented, mixed functional areas with two types of oriented facilities, mixed functional areas with three types of oriented facilities, and balanced mixed functional areas. Finally, population density, distance to the city center, land price, and cumulative opportunity accessibility of several commercial-oriented facilities were important factors affecting the existence of mixed function of urban public service facilities in Beijing. Our findings provide insights for both urban functional studies and spatial optimization of urban public service facilities in Beijing.