Objective: To understand the microbial pollution of indoor air in colleges and universities, and to provide a reference for controlling microbial pollution and improving indoor air quality. Methods: In November 2018, the air microorganisms in different functional areas and at different times were sampled and cultured in a university in Beijing by Anderson’s six-level sampler. Results: During the test, there were differences in microbial aerosol concentrations in different functional areas and at different times(F=3.99，7.77, P<0.05), the concentration of bacteria in toilets was relatively high, the average concentration was 659 CFU/m3 and the concentration of fungi in the campus was relatively high, the average concentration was 660 CFU/m3; the concentration of bacteria and fungi in the air at 12 o’clock noon was relatively low, the average concentration was 320 CFU/m3 and 322 CFU/m3. The particle size distribution of microorganisms in the air presents roughly the same law, and the peak concentration of microbial concentrations occurs at the Ⅳ and Ⅴ levels (1.1-3.3 μm), in which the highest concentration of bacteria (253 CFU/m3) appears at the Ⅴ level, and the highest concentration of fungi (249 CFU/m3) appears at the Ⅳ level. Conclusion The microbial concentration in the air of colleges and universities was mainly concentrated in the size of 1.1-3.3 μm, and it was necessary to take targeted comprehensive control measures to effectively improve air quality.