The concentration distributions of CO2 and particles are simulated by employing an numerical code in an central air-conditioning room with top-top air supply systems. Considering both of CO2 and particle concentration distributions in the zone of occupants’ breathing, the results show that indoor air quality is more acceptable under ceiling air supply mode comparing with sidewall air supply, and air supply direction can only affect the distribution of coarse particles indoor. When low-efficiency air filters are used in air handing units, the variations of indoor aerosol concentrations are simulated in two typical cities, which located in northern and southern of China, respectively. The result indicates that ceiling air supply is more suitable in northern cities as coarse aerosol are the major parts of the incoming air, and the direction of air supply has no effect on indoor particle distribution in southern cities for particle concentration in the fresh air is primarily determined by fine aerosols.