Abstract: Abstract: Plastic mulch has been widely used in agricultural production, especially in arid and semiarid regions. The study of crop evapotranspiration and crop coefficient in fields with plastic mulch is the base for irrigation scheduling and improving water use efficiency, especially in Northwest China where water shortage happens frequently. Based on field experiment data of spring wheat in 2014-2015 in the Shiyang River Basin Experiment station in Gansu province, water balance components over the spring wheat field was analyzed using the dual crop coefficient approach for crop evapotranspiration and a field water balance model through calibration and verification. The pattern and structure of water consumption under different irrigation treatment with and without mulch was revealed by simulating the variation of soil water content in the whole growing stage of spring wheat. The results showed that evapotranspiration of spring field with mulch was reduced by 10% to 16% for different irrigation treatment compared with field without mulch, which indicated the effect of water saving for plastic mulch. The plastic mulch attained the optimal effect of saving water at the initial stage and reduced the water consumption by 69.03% in the stage compared to no mulch. At the development stage, the water consumption and its rate was similar for spring wheat with or without mulch. After stepping into mid stage, plastic mulch achieved a similar evapotranspiration rate due to the higher crop transpiration. According to the analysis for water supply and consumption, the water scarcity period was at the mid or late stage for spring wheat with mulch and initial or development stage for spring wheat without mulch. Hence irrigating late properly could be advantage for plastic mulch. In the study, the calibrated basal crop coefficients for initial, mid and late stage were separately 0.101, 0.751 and 0.300 for spring wheat without mulch and 0.150, 0.900 and 0.700 for spring wheat with mulch. The basal crop coefficient in mid stage was relatively lower in this research compared to the recommended value in FAO-56 due to the difference of variety, district and climate. The larger basal crop coefficient for plastic mulch led to a higher transpiration of spring wheat in the whole stage, which adjusted the structure of evapotranspiration efficiently. The fractions of transpiration to evapotranspiration were 43%-47% and 70%-74% for spring wheat without and with mulch which meant that plastic mulch increased the fraction by 25%-27%.The evaporation was relatively stable for plastic mulch treatment due to the effect of saving moisture of the topsoil layer and limitation of the vent holes. Through the analysis of field water balance, it was seen that amount of soil water was wasted for the exorbitant initial soil water content especially for plastic mulch field. Therefore, the irrigation amount before sowing should be controlled to avoid the waste of water. Moreover, plastic mulch had an effect on influencing the growth of spring wheat. In the previous growth stage, plastic mulch can promote the rate of growth and shorten the initial stage for seven days; while in the middle and late stages, plastic mulch can prolong the length of middle stage for 10 days and delayed canopy senescence.