With 45 plots of Simao Pine (Pinus kesiya var. langbianensis) natural forests in Pu'er City of Yunnan Province,China,Shannon index (H), Simpson index (D) and Gini coefficient (G) for overall stand,Simao pine and other trees in plots were selected to quantify the stand diameter size diversity. Then,canonical correlation analysis (CCA) was used to analyze the change law of stand diameter size diversity along with climate factors,topographical factors,soil factors and stand factors. The result showed that: (1) Shannon index (H) and Simpson index (D) of overall stand and Simao pine had more consistent changes,while the H_o,D_o and G_o of other trees had great fluctuation than others which reflect that Simao pine had more abundant size classes. (2) The first ordination axis of the climate,topography,soil and stand factors reflected 58.6%,92.0%,64.4% and 76.4% for the variation,respectively,and it had better explanation on the variation of diameter size diversity along with the environment factors. (3) It was vividly depicted on the correlation between environment factors and ordination axes,the correlation between eight precipitation factors (except Annual Precipitation) and ordination axes was more than 0.15,and the correlation coefficient were not significant between temperature factors and ordination axes. The altitude (Alt) had the highest correlation with the first axis which was -0.467 5; the correlation between total phosphorus (TP), total potassium (TK), and the first axis was more than 0.30,while the stand dominant height (Ht) was the most relevant (-0.418) stand factor. (4) Simao pine natural forest had the largest H_t,H_s,D_t and D_s when the stand was under higher temperature and moderate precipitation,moderate altitude and slope,moderate total phosphorus (TP), total nitrogen (TN), organic matter (OM) content and pH value of soil,and least stand dominant height,stand average height and medium stand density index. But the change of H_o,D_o and three Gini coefficients were with irregularity.