Abstract: In order to further study the soil water and fertilizer effect of agroforestry system and guide species selection, spacing form, tillage and fertilize. A walnut-soybean intercropping system in the western Shanxi Province of Loess Plateau was used as material. Distribution of niche and underground competition characteristics of roots in a walnut-soybean intercropping system were examined by stratified digging method. The results showed that niche breadths of walnut roots were higher than that of soybean in plots 1.5-3.0 m from the tree row but it was the opposite situation in plot 3.0-3.5 m, niche breadth of roots for both walnut and soybean were decreased with the depth except for that of soybean in plot 1.5-2.0 m, niche overlap of soybean roots were lower than that of walnut in plots 1.5-3.0 m from the tree row beside the plot 3.0-3.5 m. Both niche similarity of roots and underground species competitive index between walnut and soybean were greater in the 1.0-2.5 m from the tree row, where is the main area competing for soil moisture and nutrients.