Abstract: It was intended to find the critical factors which influence understorey woody diversity of Eucalyptus plantation on the basis of comparative analysis of annual rainfall,the water content of soil and available N,P,K of soil with the method of outdoor’s sample surveying in Hainan Island.The results of compound factors correlation analysis show that,on the principal components analysis of critical factors which influence undergrowth diversity,annual rainfall is the first main factor(limiting factor)influencing on the undergrowth diversity,water content of soil follows,and then available N of soil.The results still show that the Eucalyptus fast plantations are not fit to plant in dry area where annual rainfall is less than 1 000 mm in Hainan Island based on the analysis.