Abstract: Taking 48 plots of Pinus densata natural forest as research objects, the Shannon-Wiener Index and Simpson's diversity Index were used to quantify the overall and diameter size diversity of Pinus densata stands. Correlation analysis, general regression analysis and CCA ranking analysis were used to analyze the variation of diameter size diversity index of Pinus densata forest with stand factors, topographical factors, and understory vegetation factors. The results showed that: (1) when the dominant tree species had a larger composition, the diameter size diversity was consistent in the overall stand population and dominant tree species. (2) within the sample plot in the study area, the overall diameter size diversity was generally higher than that of Pinus densata. Other tree species had a certain influence on the diameter size diversity of Pinus densata. (3) the influence of environmental factors on the stand diameter size diversity was as follows: stand factor > topographical factor > understory vegetation factor. The average diameter at breast height of stand had the highest influence on the diameter size diversity of Pinus densata.. The maximum correlation coefficient could reach 0.756, and the fitting precision was the best, R² could reach 0.6989. When CCA sort axis analysis was usd, the performance in the first axis was the best.