Abstract: Based on three years of observation on the spatio-temporal changes of negative air ion concentrations in Beijing air quality was assessed using ion polarity ratio and air ion assessment index （CI）.The results show that air cleanness degree increases gradually from city center to suburb and exurb.The air quality is best in summer and worst in winter.The air cleanness degree of forested area is better than that of non-forest area. Broad-leaved forests play a better role than coniferous forests in spring and summer for cleaning air but the opposite is true in autumn and winter.Its annual average in coniferous forests is better than that in broadleaved forests.Its seasonal difference in coniferous forests is not obvisous but in broad-leaved forests it is obviously in summer better than in other seasons.Flowing water such as waterfall and rivulet plays a significant role in cleaning ambient air.