Abstract: Based on comparative studying organ growth indicators of Robinia pseudoacacia plantation under different site conditions in Weibei Loess Plateau and 126 analytic trees biomass data, this paper analyzed the growth and biomass of R. pseudoacacia plantations in different densities, investigated the density variation characteristics of different diameter classes, which were used to fit the growth model of individual biomass related with diameter at breast height and tree height. The results showed that: after planted, with forest age increasing and density changes, the trunk, branches and roots accounted for 47.1%, 22.22% and 25.41% of R. pseudoacacia biomass respectively no matter in the abandoned land, sloping land or wasteland. In addition, leaves and fruits had the lowest content and trunk biomass in high-density stands was less than that in low-density stands. Meanwhile, when planting density of 30 years abandoned land was 1050plants/hm2, 24cm diameter class of stem biomass accounted for 65.5%. When planting density of 35 years slope farmland was 950plants/hm2, 24cm diameter class trunk biomass accounted for 58.39%. When planting density of 30 years wasteland was 1410plants/hm2, 24cm diameter class trunk biomass accounted for 51.6%. In addition, the 30-year-old trees with mid and small diameter class accounted for 67.62% of the total stand density, and the 35-year-old trees with mid diameter class accounted for 52.84% of the total stand density. Therefore, on the basis of R. pseudoacacia plantations growth status analysis results, the most suitable density of R. pseudoacacia plantations in Weibei Loess Plateau was 950-1050plants/hm2. Also, reasonable thinning of mid diameter class of 30-35 years R. pseudoacacia plantations could achieve a higher biomass yield in this area.