Abstract: Using soil moisture sensors, micro-lysimeters and eddy covariance systems，dynamics of soil moisture and evaporation, farmland evapotranspiration, and growth of rain-fed spring maize were monitored continuously in plastic mulching and un-mulching fields, and the changing regularities of soil evaporation and farmland evapotranspiration also were analyzed to explore the mechanism of plastic mulching enhancing maize yield and water use efficiency. Results suggested that, in comparison with un-mulching, plastic mulching treatment had seedling emergence rate of 99%, which was significantly higher than 80% of the former, in addition with the help of plastic mulching, each growth stage of spring maize emerged an average of 7 days in advance, meanwhile whole growth period shortened 11 days. The temperature and volumetric water content of plastic mulching soil surface layer increased by 4.9% and 19.5% respectively. Except the late growth stage, the leaf area index (LAI), plant height, aboveground and underground dry matter of plastic mulching treatment significantly were higher than un-mulching treatment, while a month after sowing by 110.2%, 13.5%, 42.9% and 12.7% respectively. Before jointing stage and from August to mature stage, the evapotranspiration of plastic mulching treatment is lower than non-mulching treatment by 6.8% and 0.4% respectively, and between June and August, the evapotranspiration of plastic mulching treatment is higher than the latter by 5.1%. Finally, the difference of average grain weight and biomass per plant in these two treatments is not significant; however, because plastic mulching improved the emergence rate significantly, the biomass and seed yield per unit area increased by 23.7% and 15.3% respectively. At the same time, the plastic mulching also markedly improved crop water use efficiency by 22.6% to 31.3 kg/(hm2·mm). It is thus clear that plastic mulching significantly reduced soil evaporation and evapotranspiration, retained soil water and heat to promote maize germination, advanced growth stages, increased daily transpiration and dry matter accumulation rate by higher increments during middle growth stage, and eventually significantly improved biomass and economic yield per unit area and crop water use efficiency.