Recently, the interaction between particles retained on HVAC and indoor air quality has gained more attention due to their possible relationship to irritation, health outcomes, and odors. The purpose of this study was to offer some reference data, contributing to a insights into particle and bioaerosols deposited in real HVAC systems. The HVAC system sanitary condition in a high-rise building in Xian, China was investigated. The deposition dust was quantified and the accumulated microorganisms (including bacteria and fungi) were identified by morphology method in different parts of central air-conditioning system. Results revealed that the average size of the particles of dust fell into the range of 620ìm, with the exception of the fresh air section. Secondly, the loading rates for the return air duct may quickly than the supply air duct for the buildings with a lot of indoor sources. We should give more attention to return air duct when arrange HVAC systems cleaning schedules. Thirdly, the index of microorganisms was more sensitive than deposition dust because the microorganism concentration already had exceed the threshold, while the deposition dust were still under the limiting value. Lastly, in the process of controlling microorganisms, it is more effective to perishing the bacteria due to always higher concentration than fungi in the same parts.