Water is a primary resource limiting terrestrial biological activity, particularly in arid and semi-arid regions. In this study, a temporal and spatial variation of water status in soil and Juglans regia and Vigna radiata during spring and summer in an agroforestry system in the rocky mountainous area of north China (a semi-arid region) was traced by analyzing the difference of hydrogen isotope. Root dynamics of the two species were measured in both dry and wet seasons. Variation in δD of was compared to detect the water sources and use. Results showed that 61.61% of J regia roots co-existed with total roots of V radiate in shallow soil (0—30 cm depth). There was a significant difference of shallow soil water content and the δD value between dry and wet season. A significantly inverse correlation (R2=0.77，P=0.02) was found between soil water content and δD value in dry season but not （R2=0.03，P=0.73）in wet season. Analyses of δD value showed that 38.39% of the total roots, distributed in the deep soil layers (30-80 cm depth), absorbed over 51% of the total water from the soil in dry season. The shallow soil provided all water in the monocropped V radiate, whereas around 1.58% to 5.39% water in the intercropped V radiate was derived from the deep soil. In wet season, around 63.52% to 78.99% water of J regia and all water of V radiate were extracted from the shallow soil, which reflected that the shallow soil was able to synchronously meet the water need for J regia and V radiate when the shallow soil had sufficient moisture. However, photosynthetic rate of the intercropped V radiate was significant (P<0.05) lower than that of the monocropped due to light competition in the agroforestry system. In dry season, predawn leaf water potential and photosynthetic rate of V radiate, as well as the water content in 0-20 cm soil layers, in the intercropping field were significant higher than those in the monocropping field. The δD data also suggested approximately 1.58% to 5.39% water in the intercropped V radiate was derived from the hydraulic lift by J regia at night. In general, the intercropped V radiate presented better water status and hence higher assimilation rate than the monocropped V radiate in dry season likely due to water lift by J regia and relative higher moisture in understory of J regia. However, the intercropped V radiate lost a certain photosynthetic capacity in wet season due to being partially shaded by J regia. Therefore, the intercropped V radiate was able to avoid harsh drought stress in dry season at the expenses of photosynthetic capacity in wet season. On the other hand, the intercropped V radiate did not competed with J regia for water resources largely due to their differential distribution of root systems. In dry season, J regia was able to extract water from the deep soil layers while the V radiate mainly depended on water from the shallow soil. In wet season water was not the limitation resource for the plant growth.