Using China’s provincial panel data during 2006-2019, this paper attempts to research the impacts of environmental regulation on green water efficiency. A Slacks-Based Measure-Data Envelopment Analysis (SBM-DEA) model with undesirable outputs is used to evaluate green water efficiency for thirty provinces in China. The result indicates a relatively low green water efficiency and significant difference between provinces in China. We have three main findings: (1) The efficiency of green water resources in eastern China is on the rise, and there is a necessity to make further optimization. (2) Environmental regulation has a positive effect on the national green water efficiency. Because of regional differences, environmental regulation has a significant positive effect on the green water efficiency of both the east and the west, but its impact on the western green water efficiency has not passed the significance test. Still, the negative elasticity coefficient indicates that environmental regulation is not conducive to improving the efficiency of green water resources in the west. (3) As for control variables, technological progress, education input, urbanization rate, and the level of opening up facilitate more rapid the growth of green water efficiency in the east. The roles of the level of opening up, education input, and urbanization rate in improving central green water efficiency are extremely obvious. There is a negative correlation between economic development and green water efficiency in western China. Except for the abundance of water resources and foreign capital utilization of the west, the remainder of control variables shows a positive effect.