Abstract: Cluster analysis is used to study 13 species of Pediastrum in Haikou. Twenty-four traits are selected for systematic observation and recording, and trait data are subjected for R-type clustering and principal component analysis, then Q-type clustering for Pediastrum species. The results show that most of the 24 selected traits in the R-type cluster analysis are scattered and relatively independent, indicating that the selection of various traits is reasonable. The results of principal component analysis show that 24 traits can be integrated into 7 principal components, with a cumulative contribution of 91.253%, among which the larger contribution are the width of the top and base of the outer cells, the degree of lateral wall connection between the outer cells, the sides of the outer cells, the number of outer cell horns and the length of the horns. The 13 species can be divided into 5 categories. The main character of the first category is nearly four-prism of outer cells with 2 horns; the second category has granular cell wall, outer cells with 5~6 edges and 2 horns; the third category is outer cells with 1 horns; the fourth category has nearly four-prism inner cells; the fifth category has oval outer cells. The study show that the morphology of outer and inner cell, horn and cell wall particles have an important role in the morphological classification of Pediastrum.