Canbon sequestration and O2 release due to the rapid development of urban greenland could be beneficial for the global implementation of energy saving and CO2 emission reduction, however, this poses another question that increased the demand for irrigation becomes a concern for the sustainable utilization of water resources, especially for Beijing, with the scarcity of land and water resources for Greenland. E.japonicus, as one of typical green -roof plants, has the advantages of alleviating the effect of heat island and improving microclimate environment. However, it needs to make clear that how the physiological performance of E.japonicus treated with different water stresses including full irrigation (CK) (90%-100%FC), low water stress (LWS) (75%-85%FC), moderate water stress (MWS) (65%-75%FC), and serious water stress (SWS) (50%-60%FC) is, and landscape function and ecological serves function are also considered as integrated indicators to selecting optimization irrigation strategy in this study. The results showed that the photosynthetic rate, transpiration rate, stomatal conductance and water use efficiency of E.japonicus were in the order of LWS>MWS>SWS in three treatments of water stress. Moreover, the photosynthetic rate, transpiration rate, stomatal conductance and water use efficiency under LWS were 1.55%、3.3%、4.13%、7.1% higher compared to CK, respectively. Higher leaf area and chlorophyll content were also measured under the treatment of LWS. In terms of ecological serves function, carbon sequestration and oxygen release, and cooling and humidity were lessening with the soil moisture reducing which express the positive correlation relationship. but the differences was no significant.The LWS(75%-85%FC) stimulated the growth of E.japonicus, and effectively regulated the  distribution of the assimilation object to chlorophyll and that for the growth of  leaves. Besides, it played a significant role in ecological environment. Therefore, the LWS (75%-85%FC) is the optimal water-saving irrigation model.