Outdoor PM2.5 could penetrate through fresh air and the wall gap into the room especially when outdoor PM2.5 is particularly high, which lead to the rapid rise in the indoor PM2.5 concentration. To determine the influence of the air conditioning system form on the indoor and outdoor PM2.5 concentration, the PM2.5 concentration in the room with different air conditioning forms is measured in the summer of 2015, which is in an office building in Chongqing. The measured results show that: the range of the indoor and outdoor PM2.5 concentration ratio is 0.59～0.76,0.47～0.76,and 0.71～0.91, respectively when the air conditioning system is. The relationship of indoor and outdoor PM2.5 concentration correlation coefficient is centralized air conditioning environment(0.94) >non air conditioning environment (0.92) > decentralized air conditioning environment (0.77).The results show that air conditioning form in office building has the effect of on indoor PM2.5 concentration.