Based on the measured δ¹⁸O in soil water and precipitat ion in the middle of the Tibetan Plateau in the summer of 1998, relationship between δ¹⁸O in soil water and soil water movement is discussed in this paper.The results show that δ¹⁸O in soil water in the upper layer is affected directly by δ¹⁸O in precipitation, and both vary in the same trend, whereas δ¹⁸O in the underground water is not much affected by δ¹⁸O in precipitation directly, and it varies slightly.This indicates that the underground water there is not replenished from precipitat ion in the current summer, but from accumulation of severa-l years precipitation.The variation of δ¹⁸O in different layers of the profile soil reflects the movement of precipitation water from surface towards bottom in the soil.The impact of precipitation on δ¹⁸O in soil water is obvious only in the water of the upper soil layer, while the underground water has more impact on δ¹⁸O in soil water of the lower layer, showing the important role of ground water in soil water movement.