Abstract: Pinus kesiya var. langbianensis natural forests in Pu′er City Yunnan Province were taken as study objects. Forty-five plots in 3 regions were selected. Standing forest diameter size diversity of all the trees, P.kesiya var. langbianensis and other trees in plots were all calculated through 3 indexes which were Shannon Index (H), Simpson Index (D) and Gink coefficient of sectional area (G). Regional differences were tested and analyzed through Factor Analysis of Variance and Tukey test. The change regulation of standing forest diameter size diversity were analyzed through Canonical Correspondence Analysis (CCA). The change regulation was influenced by 4 environmental factors. They were climate, topography, soil and standing forest. The results showed that a difference of standing forest diameter size diversity index exists in 3 regions (M, P, L) partly. Topography, soil and standing forest were the main factors which caused the regional differences. Therefore, differences of standing forest diameter size diversity exist in different regions. Regional environmental heterogeneity had influence on standing forest diameter structure in the sample plot.