Abstract: In order to protect the ecological phreatic level in ecologically fragile mining area,the reasonable thickness of protected loess with mining were studied. Based on groundwater dynamics,the mechanisms of phreatic level leaky with mining had been analyzed. Also,the laws of phreatic level variation with different mining residual thickness and the physical properties of loess had been analyzed by water-electric similar simulations. The results show that the more leaky head and the weaker water isolation,the smaller phreatic decline with mining. While 42. 6 m loess of Lishi Group or 21. 0 m loess of Baode Group,the mining phreatic level can range in the ecological phreatic level for the typical ge- ological condition of the study area. Water-electric similar simulations of typical geological condition in mining work- face is achieved by multiple software. The relationship of mining residual thickness and phreatic decline shows that wa- ter-electric simulation results are anastomosing in theoretical results. The results are preliminarily proved by engineer- ing practice.