Distribution of root biomass in different diameter classes of root system is an important structural feature of root. By using digging method, composition and spatial distribution of Populus root biomass were studied in the Dawenhe Watershed. The results showed that: 1) there was a significant difference in the biomass among varied root diameters. Among them, the biomass of 10 -20 mm roots was the biggest, followed by that of roots less than 2 mm, and the biomass of roots >20 mm was the smallest. 2) Biomass spatial distribution pattern of the roots with different diameters were basically identical. On horizontal direction, the root biomass was significantly different among all layers. The farther from the trunk, the smaller the root biomass. The difference in distribution of root biomass along vertical direction was also very remarkable, but without evident order. 3) With soil depth increasing, soil bulk density, soil organic matter and available N, P, K contents decreased in an insignificant manner. The spatial distribution of Populus roots had a weakly positive correlation with soil nutrient.