At present, the mechanism research on glacial lake outburst mainly focuses on the ice quake and ice landslide, etc. To some glacial lakes, the seepage deformation is the dominant factor in outburst process. Taking the Yindapu Glacial Lake in Tibet as an example, using SEEP/W module of FEM software ( GEO-STUDIO) , the authors analyzed seepage stability of terminalmoraine ridge dam. The leading role of seepage deformation in some glacial lake outburst mechanism is proposed and proved.