Abstract: To investigate the effects of stand density on the crown structure and understory vegetation of 25-year-old Pinus massoniana plantation, experimental plots with different planting densities were selected in Zhenlong Forest Farm, Hengzhou City, Guangxi Zhuang Autonomous Region. Single-factor analysis of variance(one-way ANOVA), Pearson correlation analysis, and redundancy analysis were employed to explore the influence of stand density on the growth, crown structure, and understory herbaceous diversity of P. massoniana. The results are as follows the stand density was in 2500 trees/ha⁻¹–6000 trees/ha⁻¹. With the increase of planting density, the canopy structure LAI of P. massoniana plantation community increased first and then remained unchanged, and gradually stabilized at DS₂(3300 trees/ha⁻¹). The scattered radiation and direct radiation of understory illumination were suitable for DS₂. The understory herbaceous diversity indices, including the Simpson index, Shannon-Wiener index, Pielou evenness index, and Patrick index, showed no significant changes with variations in stand density in the P. massoniana plantation. However, there was a highly significant negative correlation between canopy openness of the community crown layer and the Shannon-Wiener diversity index of understory herbaceous vegetation. In summary, the canopy structure of P. massoniana plantation community was better under the stand density of 3300 trees/ha⁻¹, and the scattered radiation and direct radiation under the forest were conducive to the maintenance of undergrowth vegetation diversity, which was the reasonable forest density of P. massoniana plantations.