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• The mulching film is conducive to field drainage after excessive water input. • The mulching film reduces loss of soil water and improves water use efficiency. • The irrigation amount is assessed based on whether it is covered by plastic film. In arid areas, where water resources are extremely scarce, the consumption of agricultural water resources is generally higher than 80% of the total water resources. Reasonable irrigation practices and effective water conservation methods are critical to the efficient utilization of water resources. In order to analyze the rationality of existing irrigation mode and evaluate the effect of plastic mulch on irrigation water migration, this research monitored the hydrogen and oxygen stable isotopic composition of soil water in Minqin oasis farmland located in the arid area of Northwest China. We used the soil water balance equation and the Craig-Gordon model to quantify the infiltration and evaporation of soil water. The result shows: (1) After irrigations in the corn growing season, the average irrigation water of 29.9 ± 4.2% (29.0 ± 6.7 mm), 29.6 ± 2.9% (28.4 ± 2.7 mm) and 30.3 ± 4.3% (29.0 ± 3.5 mm) penetrated into the soil layer of 0–10 cm, 10–50 cm, and 50–100 cm respectively. The field drainage under plastic film mulching was higher than that of no film mulching. (2) Plastic film mulching is the main reason for the difference in SWlc-excess of the surface layer. The evaporative loss of 0–5 cm soil water in non-mulched soil was generally higher than that of plastic-mulched soil. During the whole corn growing season, plastic film mulching effectively reduces evaporation losses of surface soil water and improves water use efficiency. The results of the reserch are very useful for understanding the hydrological process of agricultural lands in arid oases, improving agricultural irrigation water utilization rates, and optimizing water conservation models. [ABSTRACT FROM AUTHOR]