Soil water content in arid and semiarid area is very low so that the amount of soil water extracted from soil is often not enough to determine its stable isotope δD and δ¹⁸O. In this study, a desert sandy soil and a farmland sandy soil were collected from the semiarid areas, and mixed water samples were extracted from the soils and ultrapure water under different proportions. We found that isotope δD or δ¹⁸O values of soil water, mixing water, ultrapure water showed very significant linear correlation (R²>0.99, P<0.001). The measured and theoretically predicted isotopic data by the End-member mixing model were consistent R²>0.99, P<0.001, which was not affected by soil texture and organic C content. This study provides a new method for obtaining stable isotope δD and δ18O of soil water even if it is quite low, and can be used in arid and semiarid areas.