To investigate the regulatory effect of Castanopsis hystrix and Pinus massoniana mixed planting on the diversity of understory plants, experiments with different mixed planting ratios and pure forest control were set up at the state-owned Liuwan Forest Farm in Guangxi Zhuang Autonomous Region. The results showed that the mixed planting treatment significantly enhanced the diversity of understory plants. Among them, the Shannon index of the shrub layer in the treatment with ζ (Castanopsis hystrix∶Pinus massoniana) = (3∶7)~(5∶5) was more than 62% higher than that of the pure Pinus massoniana forest, presenting a "single peak" change trend. The analysis of environmental driving mechanisms revealed that the mixed planting ratio indirectly promoted diversity enhancement by reducing canopy density, improving understory light conditions, and promoting soil nutrient accumulation (total effect 0.71). It was recommended to adopt the same-age inter-row mixed planting mode with ζ (Castanopsis hystrix∶Pinus massoniana)=(3∶7)~(5∶5) to achieve the coordinated improvement of forest growth and biodiversity.