Groundwater is an important influential factor of bank slope stability, and drainage facility for bank slope is necessary in waterway regulation project. Horizontal drainage holes could effectively reduce groundwater level by draining away the water of deep soils. The drainage effect of horizontal drainage holes in bank slope is studied by means of three-dimensional transient seepage numerical simulation. The numerical result agrees with indoor model test result. The groundwater of slope converges to horizontal drain and drains away from slope. Simulation of characteristic real bank slope shows that the drainage effect of horizontal drain for bank slope is apparent, and the control region of seepage field is enlarged. The deeper the drainage hole is, the better the drainage effect is. The research results could provide references for the application of horizontal drainage holes in waterway regulation engineering.