This paper recommends a method of lowering the ground water table by setting up the horizontal drains in hill slopes.Based on the finite element analysis, the method advances the experiential formulation about the downstream water depth of the horizontal drains and the distance between the horizontal drains. The experimental result seems to agree reasonably well with the expression.So, in engineering practices,the formulation can be used to predict the amount of lowering of the downstream water depth, and also to confirm the distance between the horizontal drains.