The rigid body limit equilibrium method is widely used in study on the stability against sliding of gravity dam subjected to earthquake along weak strata in rock foundation .In the study process it was found that the determination of the resisting force angle γ provided by the resisting body and the sliding angle β of the resisting body are extremely important to the reliability of analysis results .The principle determining both angles is put forwarded in this paper. The maximum value of the resisting force angle should be smaller than the included angle of the principle stress direction on the vertical plane in foundation at dam toe and horizontal plane under basical loading; the sliding angle of the resisting body should be correspongding to the minimum value of the resisting force in the equilibrium equation .The stability against sliding of the Tingzikou gravity dam along weak strata in rock foundation is studied based on the principles mentioned above.