[Objective] Studying the effect of plantation density on understory plant diversity of Haloxylon persicum, to give support for ecological function and technical management of Haloxylon persicum.[Methods] Five types (1 250, 1 111, 1 000, 1 333, 1 667 plants/hm²) of 10~15 years' stand density were selected, and indices of important values, species richness, species diversity index, evenness were calculated to study the relationship between stand density and plant diversity of Haloxylon ammodendron plantation in Minqin County, Gansu Province.[Results] ① There were 16 vascular plants in 15 families and 9 genera of the under-growths in Haloxylon ammodendron forest. Among them, there were 6 families, 6 genera and 6 species in shrub layer, and 4 families, 7 genera and 10 species in herb layer. ② The dominant species of shrub layer changed obviously with the variation of plantation density, while the dominant species of herb layer were hardly influenced by density. ③ Plant diversity of understory vegetation responded non-linearly to stand density. As stand density increased, plants diversity increased initially and decreased laterally. ④ Comprehensive evaluation showed, when density arrived at 1 250 plants/hm², the understory plant diversity reached the maximum. At that plantation density, the growth and development of understory vegetation performed well.[Conclusion] The response of plant diversity of understory vegetation to stand density was non-linear. When the density arrived at 1 250 plants/hm², the density was more appropriate for the growth and development of understory vegetation.