In order to explore the differences of understory plant diversity in different forest communities in Shenmulei, the typical sample plot method was used to study the five major forest communities in Shenmulei Forest Area of Jiajin Mountain: Picea asperata forest, Picea likiangensis forest, Larix potaninii forest, coniferous and broad-leaved mixed forest and broad-leaved forest. The species composition and species diversity of understory plants in different forest communities were compared, and the redundancy analysis of stand factors and understory plant diversity was carried out to determine the main stand factors affecting understory plant diversity, which could provide theoretical basis for local forest management. The results showed that: (1) 147 species of understory plants belonging to 61 families and 108 genera, were recorded in the study; the families, genera and species composition of understory plants in Picea asperata forest community was the richest. (2) The H, H", D and JSW values of all types of communities were: herb layer > shrub layer; Picea asperata forest community had the highest species diversity in shrub layer, Picea likiangensis forest community had the highest species diversity in herb layer, and the diversity of understory plant species in coniferous and broad-leaved mixed forest community and broad-leaved forest community was poor. (3) The average under branch height and stand density were the main stand factors affecting the species diversity of shrub layer (P<0.01). The average under branch height was negatively correlated with the D value, H value and H" value of shrub layer, and the stand density was positively correlated with the four diversity indexes of shrub layer. The average under branch height was the main stand factor affecting the species diversity of herb layer (P<0.01). The average under branch height was positively correlated with the H value, H" value and JSW value of herb layer. This study suggests that the species diversity of understory plants in Picea asperata forest community and Picea likiangensis forest community is high. The average under branch height and stand density are the main stand factors affecting the diversity of understory plants in different forest communities in Shenmulei.