The method of the stability calculation of landslide with pore pressure is presented and an amelioration for the imbalance force method is done. A three-dimensional numerical simulation with FLAC3d has been carried out for a landslide of a hydropower station to study the pore pressure

phreatic level and flow discharge

which affected by the subhorizontal drainage holes

during the drainage period. The research of the stability change of the landslide is accomplished by the amelioration imbalance force method with the data achieved from the simulation results of FLAC3d with a program written by FISH language. The results indicate that the procedure of the stability expansion is represented properly by the amelioration method. The degrees of increased stability and the decreased imbalance force are underestimated with normal methods during the drainage period.