Abstract: To investigate the seasonal and spatial variations of WSII in the plateau city of Kunming,the PM2.5 samples have been collected at three sites from April 2013 to May 2014.The PM2.5 and WSII have been analyzed,and the main source of WSII have been apportioned by combining the meteorological factors,sulfur oxidation ratio,nitrogen oxidation ratio and principal component analysis.Results show that the concentration of PM2.5 presents seasonal trend with the highest values in spring((105.9±48.0)μg/m3),followed by winter ((92.7±51.6)μg/m3) and autumn ((74.7±41.4)μg/m3),then summer((72.2±30.3)μg/m3).Seasonal variations of TWSII concentration is with higher values in summer followed by winter,and lower in spring and autumn,with concentration of(38.0±18.3)μg/m3,(22.0±11.4)μg/m3,(18.4±4.8)μg/m3 and (13.6±3.1)μg/m3 respectively.SO42-、Ca2+、NO3-、NH44+ are the dominant ions in WSII,which account for 27.7%,17.8%,15.2% and 9.5% respectively.Secondary ions concentration is 13.9μg/m3,accounting for 16.5% of the mass concentration of PM2.5,which indicates less secondary ions in the air of plateau Kunming City.The ratio of NO3-/SO42- is between 0.21 and 0.68,indicating that stationary source is the dominant source contribution.The results of principal component analysis suggest that WSII mainly come from mixed source with soil dust and construction dust,coal-fired source and industrial process source.