In order to investigate the impact factors and mechanism of high water use efficiency under water-saving irrigation technology, experiment with two irrigation treatments was carried out in rice field, including flooding irrigation (FI) and nonflooding controlled irrigation (NFI). In the FI rice fields, a depth of 3~5cm standing water was always maintained after transplantation, except during the drainage period in later tillering and yellow maturity stages. In the NFI rice fields, the pond water was kept between 5mm and 25mm during the first 7~8d after transplantation at the regreening stage. At other stages, irrigation was applied only to keep the soil moist and flooding was avoided;standing water up to 5cm depth in NFI fields was maintained for less than 5d just to meet the requirements for the pesticide or fertilizer application. The relationships between stomatal regulation, environmental factors and leaf water use efficiency were studied, meanwhile, the regression equations of leaf water use efficiency were established, and the path analysis method was applied to analyze the impact factors. The results showed that there was a quadratic regression equation between stomatal conductance (Gs) and transpiration rate (Tr), photosynthetic rate (Pn), leaf water use efficiency (LWUE) under NFI treatment, to maintain high LWUE, optimal stomatal conductance was 0.54mol/(m2·s), and the peak value was appeared earlier than that of FI treatment. There was also a quadratic regression equation between LWUE and environmental factors, including air temperature (Ta), leaf temperature (Tl), leaf-air temperature difference (ΔT) , air CO2 concentration (Ca) and photosynthesis available radiation (Par). While LWUE was negatively related to intercellular CO2 concentration (Ci) and positively correlated with soil moisture (θ), the relationship between LWUE and relative humidity (Rh) was exponential. The temperature factors composed of Ta, Tl and ΔT contributed 39.19% to LWUE，while the CO2 concentration factors composed of Ca and Ci contributed 17.81%, the vapor factor composed of Rh and θ contributed 17.81%, and the light factor composed of Par contributed 9.01%. Furthermore, the regression equation of LWUE was established, and the path analysis method was applied to analyze the impact factors, as for the NFI treatment, it was found that Par, Gs and θ may not be the main influence factors, the sensitive indicators affecting the LWUE were Ci, Tl and Rh.