Abstract: The species of understory vegetation in different mixed plantations, the biodiversity, biomass and growth status under different salt content in the soil in the salinealkali area in Yellow River delta were studied by sample plot survey. Totally 31 understory plant species were recorded, which belonged to 31 genera and 14 families. Among which, there were 10 species from gramineous family, accounting for the largest proportion. The dominant species of understory vegetation in 1a, 3a, 7a and 22a mixed plantations were Phragmites australis + Artemisia selengensis,Ixeris polycephala + Chenopodium album + Setaira viridis, Plantago asiatica, and Cleistogenes squarrosa respectively. There were few understory vegetation species with low biodiversity, less enriched species and more sparse species. The aboveground biomass of the understory vegetation species was generally greater than that of the underground. The dominate species in the mixed plantations with high salt content were mainly the salt tolerant plant species, and vice versa. Because of the strong adaptability to different environmental conditions, Ixeris polycephala and Chenopodium album grew well on sites with four different salt content levels.