Based on sampleplot survey to the Populus euphratica in the lower reaches of Tarim River, we established the model between biomass and factors such as DBH and height; Besides, the distribution of biomass of P. euphratica was analyzed in the district; Finally, the total biomass was assessed based on the area of P. euphratica forest in the lower reaches of Tarim River. The results are as follows: (1) biomass model of P. euphratica: B=128.09(D²h)^0.6318（R²=0.892），which was high in precision, and the discrepancy between measured value and simulated value was nonsignificant (P>0.05), thus the model could be used to assess the biomass of P. euphratica. (2) The biomass presented a trend that the biomass declining was relevant to the distance from river; P. euphratica distributed within 700 m of river; and the biomass of this area accounted for 91.37% of total. (3) The biomass per unit area was 416.613 kg·hm^-2 in the lower reaches of Tarim river and the total biomass was 2.474×10⁴ t.