Since the year of 2011, PM_2.5 have become a heated topic in China. Particulate matter (PM), also known as aerosol, is one of the major pollutants that affect air quality. Exposure to particular matter with aerodynamic diameters less than 2.5 μm (PM_2.5) can cause lung and respiratory diseases and even premature deaths. In this paper we use the aerosol optical depth (AOD) retrieved by the Synergetic Retrieval of Aerosol Properties (SRAP) method from MODIS data to calculate PM_2.5., then estimate number of days with good air quality (PM_2.5≤0.075 mg/m³) at each pixel over China in 2008. The result is applied to examine the air quality, From which we can see that there are about 200 days with good air quality in Beijing, in agreement with Official Reports. Throng analyzing the calculated days with good air quality in August from 2005 to 2008, we examined the temporal variations of PM_2.5 over China, These findings indicate a positive annual variation trend before Beijing 2008 Olympic Games, however the air qulity of Beijing still needs improving.