In order to get a profile of indoor air condition in large public buildings in Chongqing, and simultaneously get a betterunderstanding of main factors that affect indoor environment, a big field study has been conducted in five large public buildingsrandomly selected in Chongqing. As part of the study, real-time data of indoor PM2.5 concentration for 23 rooms in three officebuildings and 5 floors in two commercial buildings have been collected, together with real-time data about outdoor PM2.5concentration for the five buildings. During the test, room or floor information has been recorded by the investigators, includingroom sizes, opening or closing of doors and windows, operation of air conditioning systems, office devices, number of occupants,and activities of occupants.etc. Results indicated that in spite of the big differences of indoor PM2.5 concentration in differentbuildings, the overall indoor PM2.5 level is very high and the indoor PM2.5 pollution is serious. The mean value of indoor PM2.5concentration in office buildings is 181μg/m3, and that in commercial buildings is 96μg/m3. Outdoor PM2.5 concentration is to a largeextent has a big influence on indoor PM2.5 concentration. Results from statistical analysis show that indoor PM2.5 concentration issignificantly associated with outdoor PM2.5 concentration, with r=0.917 and P-value＜0.001, which matches with studies in othercities very well. Arainy day will contribute to a much lower indoor PM2.5 level. The I/O value which means the ratio of indoor PM2.5concentration to outdoor PM2.5 concentration ranges from 0.41 to 3.76, within which the I/O values of 4 office rooms and 4 floors arebigger than 1, and the other 22 rooms and 1 floor are much smaller, indicating that resources of PM2.5 pollution may exist in indoorenvironment. This phenomenon is especially outstanding in commercial buildings. Further analysis found that the level of indoorPM2.5 in rooms with smokers or smoking smells is much higher than rooms without. Except for outdoor PM2.5 concentration, indoorsmoking can be important factors that having great influences on indoor PM2.5 level in public buildings.