To investigate the microorganism ecological distribution and contamination mechanism in supply air ducts, a series of experiments for testing microbial counts and identifying the microorganism species were conducted under different cases. The results showed that Gram-positive accounted for 56% while Gram-negative for 44% for all bacteria which were made up of Cocci and Bacillus. The detective fungi contained Penicillium, Aspergillus, Cladosporium, Alternaria, Mucor, and Trichoderma, which accounts for 30.1%, 17.2%, 41.8%. 5.6%, 2.8%, 0.5%, respectively and the predominant fungi were Penicillium, Aspergillus and Cladosporium. The supply air velocity had a great influence on the dust distribution in whole test facility. The microorganism counts and deposition dust were positively correlated, when deposition dust increased 1 times, the microorganism counts could maximally increase 1-2 times, and the bacteria correlation coefficient was more significant than fungi (R-bacteria > R-fungi). The microorganism counts declined with relative humidity and temperature increasing, and the bacteria were more susceptible to the relative humidity and temperature than fungi (R-bacteria > R-fungi). Orthogonal experiment design indicated the main factor of the microbial growth was temperature: but at supply air velocity of 3.0 m/s, the relative humidity was the main influence factor to the fungal growth. Crown Copyright (C) 2011 Published by Elsevier B.V. All rights reserved.