Abstract: The concept of relative importance to the information from the monitoring values of single pollutant was used to select the optimal numbers of environmental monitoring sites,and formula was presented for comprehensive relative importance of multi－pollutant.On the basis of comprehensive relative importance of information in sites,the optimization of environmental monitoring sites was carried out by principal component subset selection.This method not only reduces the calculation but also determines the comprehensive optimal sites of multi－pollutant easily.The monitoring sites selected from atmospheric environmental monitoring sites in Guiyang City can represent the atmospheric environmental quality.