[Objective] The effects of different planting densities on plant community structure, species diversity, and soil physicochemical properties of Pinus tabulaeformis plantation were studied, and the appropriate management density of soil and water conservation forest were determined to provide reference for the improvement of ecological function and sustainable development of P. tabulaeformis plantation in the arsenic sandstone area. [Methods] Taking P. tabulaeformis plantation on the slope of Xujiamen Qiangou in Xiheidai of Zhungeer Banner as the research site, the relationship between understory herbaceous plant community composition, species diversity and soil physical and chemical properties under four planting densities (950, 2 000, 3 250, 4 300 plant/hm²) was explored through field sampling and indoor investigation. [Results] ① There were 33 species from 9 families and 19 genera in the study area, and the number of plant species was the highest when the afforestation density was 3 250 plant/hm². The understory herbaceous plant communities of P. tabulaeformis were mainly Gramineae, Compositae, Fabaceae and Rosaceae. With the increase of afforestation density, the associated dominant species transited from sunny and mesophytic species to shady species. ② The species diversity index, plant height, biomass, soil organic matter, available nitrogen, phosphorus and potassium content, and soil water content in the understory herbaceous layer were 3 250 plants/hm²>4 300 plants/hm²>950 plants/hm²>2 000 plants/hm². ③ Afforestation density had significant effects on soil water content, organic matter and available nitrogen, phosphorus and potassium content (p<0.05), but had no significant effects on plant species diversity index (p>0.05). In the correlation analysis, soil organic matter, available nitrogen, phosphorus and potassium content were most closely related to the species diversity index of herbaceous layer. [Conclusion] The planting density of 3 250 plant/hm² was more conducive to the growth of understory plants and the improvement of soil fertility in the arsenic sandstone area of Zhungeer Banner.