In this study, sampling plots and control were chosen in the Beijing Olympic Forest Park(BOFP) and negative air ion(NAI) concentrations were measured continually in clear days of August 18--20, 2009. The results indicated that the effect of plant community structure on NAI was significantly different. NAI concentration of the tree-shrub-grass communities was higher than that in the simple plant communities, especially higher than that in the single-layer ones. The correlation between multiple plant communities and NAI concentration was significant. Moreover, NAI concentration of the deciduous broad-leaved forest regions was higher than those of the coniferous forest and mixed conifer and broad-leaved one. The results showed that there was positive correlations between NAI concentration and air relative humidity and air temperature.