Abstract: In this experiment, the species composition and biomass accumulation of Leguminousae species and vegetation diversity under Cupressus chengiana and Pinus radiata plantations after 20 years of afforestation were studied by typical plot method in Wenchuan and Lixian sections of the Minjiang River arid valley. The results showed that: （1） There were 41 species of shrub and grass vegetation under C. chengiana plantation, including 5 kinds of Leguminous shrub species and 2 kinds of herb species, among which Indigofera lenticellata was a relatively stable companion shrub. With the increase of altitude, Leguminosae shrubs decreased, coverage reduced, Leguminosae herbs gradually appeared, and the total biomass of Leguminosus species decreased. （2） There were 21 species of shrub and grass vegetation under P. radiata plantation, including 2 species of Leguminous shrubs. Lespedeza bicolor had obvious advantages in the shrub layer, and there were no leguminous herbs. With the increase of altitude, the number of Leguminous shrubs decreased, and the coverage and total biomass of Leguminous species changed little. （3） The understory biomass of Leguminosae species under C. chengiana plantation accounted for 45%, and there were root nodules in Indigofera lenticellata and Caragana sinica. The understory biomass of Leguminosae species under P. radiata plantation accounted for 38%, and no root nodules were found. （4） The vegetation coverage, biomass and biodiversity index of understory shrub and grass species under C. chengiana plantation were much higher than those of P. radiata plantation, and the shrub biomass can reach 20 times that of P. radiata. In general, the understory vegetation coverage of C. chengiana plantation was higher, the species diversity was higher, the Leguminous species were richer, the biomass accumulation were larger, and there was also Indigofera lenticellata which played a role in nitrogen fixation, and its resistance to drought stress and external interference was relatively strong.