For reducing indoor air pollution and controlling its effects, the method of sampling and experimental analysis is adopted. In addition, physical, chemical and microbial characteristics of the air conditioning system dust accumulation are discussed in details. And dust particle size distribution was determined, and by combustion oxidation - non dispersive infrared absorption method, carbonaceous materials were determined, including organic carbon (TOC), inorganic carbon (IC) and total carbon (TC). Strictly in accordance with the microbial culture methods in "Public Place Hygiene Standards of Air Conditioning and Ventilation System" issued by the Ministry of Health, the breeding situation of bacteria and fungi in different parts of the air conditioning system is analyzed. At last, the following conclusion are drawn through the above research: (1) The HVAC system, in addition to 6 sampling sections outside the fresh air, the average particle diameter of dust is between 6-20μm, and the average particle dust in the air segment size is 41.30μm. (2) The proportion of TOC, TC and IC is between 2%-7.5%, 2.5%-8.5% and 0.7%-1.2% respectively, in which the air supply pipe is the largest, and the fresh air section is the smallest. Dust associated bacteria are between 60000-210000CFU/g, the associated fungi are between 60000-140000CFU/g, and the microbial accumulation in the cold section is the most.