Abstract: In order to investigate a water-saving, high yield and efficient irrigation schedule for onion in arid region of Northwest China, a two-year field experiment with regulated deficit irrigation for onion has been conducted. Crop evapotranspiration and its influencing factors for onion were analyzed, crop water production function was established, and the optimal irrigation schedule for onion was determined. Results showed that the crop evapotranspiration of onion with drip irrigation under mulch of plastic film was 170.1-395.7 mm. With the increasing of irrigation and crop evapotranspiration, the onion bulb yield increased, while the water use efficiency decreased. Onion bulb yield and water use efficiency were not significantly affected by water stress during the establishment and ripening stages. The sensitive index was found the largest at the bulbification stage, followed by the development stage and relatively small at the establishment and ripening stages. Higher onion bulb yield can be obtained when the irrigation depth was up to 375 mm in drought years in arid region of Northwest China. The recommended amount of irrigation is 30-40, 130-140, 170-190 mm and 25 mm for the stages of establishment, development, bulbification and ripening, respectively, and the irrigation intervals are 5 days. The optimal irrigation schedule is expected to provide scientific implications for water-saving irrigation of onion crops in the arid region of Northwest China.