[Objective] Pinus sylvestris is considered to be one of the preferred coniferous species in the Mu Us sandland afforestation. Studying the effect of stand density on the stand structure and understory vegetation development in order to provide important support for the stabilization of sands by afforestation in this region.[Methods] We selected the Pinus sylvestris plantations of 25 years old at different densities in Yulin City, and analyzed the changes in plantation structure and understory species diversity of Pinus sylvestris plantations with variations in stand density.[Results] The results showed the stand density and average diameter followed exponential relationship. And the height to diameter ratio showed a significant positive linear correlation with stand density. A total of 16 understory plant species were recorded, belonging to 13 genera and 8 families. Gramineae of foxtail grass was dominant species. Herbal species richness and diversity index showed an obvious increasing trend with decreasing density. The stand density was negatively correlated with the species richness and diversity index. Shannon diversity index was the most sensitive to changes in density.[Conclusion] In the management of Pinus sylvestris plantations in Mu Us sandland, the understory species diversity index can be seen as an indicator of the suitability of stand density.