A theoretical and experimental measurement analysis on air filter of fan coil unit and fresh air system was conducted through establishing a mass balance equation. In this paper, a certain building in Xi'an is taken as an example, the outdoor PM2.5 concentration is calculated based on "no assurance of 10 days". The results show that the synthetic fiber of G3 or G4 air filter can be used in return air of FCU when the fan pressure is 50-80pa, and the G2 or G3 air filter can be used when the pressure is 30-50pa. Meanwhile, G4 and F7 or F8 two-stage filtration can be used in fresh air system when equivalent filtration efficiency air purifier should be installed.