[Objective] This paper studied the water use mechanism of farmland shelterbelts with important protection function under the mode of farmland water-saving irrigation, in order to provide a scientific basis for optimizing irrigation of the local farmland and farmland shelterbelt. [Methods] The stable oxygen isotope (δ¹⁸ O) was used to study the water use mechanism of Populus euphratica, Elaeagnus angustifolia and Ulmus pumila in the arid oasis areas under farmland water-saving irrigation mode. The results were compared and analyzed based on the direct judgment method, IsoSource model and water absorption depth model. [Results] The sources of water use was different for different tree species at vary time. In April, P. euphratica mainly used 10-20 cm of shallow soil water, and its contribution rate was 83.3%. E. angustifolia mainly used deep soil water at 80-120 cm and 120-160 cm depth with contribution rates of 50.6% and 16.9%, respectively. Whereas, 82.5% water usage of U. pumila was from 50-300 cm depth of soil water and groundwater. In May, 57.1% of water used by P. euphratica was from 0-30 cm shallow soil water. Surface soil water at 0-10 cm soil depth contributed 50.8% and 52.7% for the water usage of E. angustifolia and U. pumila, respectively. In June, the contribution of water source for P. euphratica usage was 38.7% from 0-20 cm shallow soil, while 10.5% from groundwater. And 76.9% of the water content of E. angustifolia came from shallow soil of 10-20 cm, 49.1% of water usage for U. pumila came from 0-80 cm soil, and another 12.3% from groundwater. The average water absorption depth of P. euphratica was 18, 28, 25 cm, respectively in April, May and June. For E. angustifolia, it was 118, 37, 34 cm, respectively. At the same time, it was 95，37，29 cm for U. pumila, respectively. [Conclusion] When selecting farmland shelterbelt tree species, while ensuring the protection benefit of shelterbelt, we should choose the species with deep soil water or groundwater as the main water source, so as to exert the protective effect of farmland shelterbelt more efficiently.