In Design code for concrete gravity dam (NB/T 35026-2014) of China, the method for anti-sliding stability with multiple sliding surfaces gives the solution of resistance-action ratio η based on the prerequisite that resistance ΔQ_i of sliding blocks meets internal force equilibrium. However, as the angles between ΔQ_i and horizontal direction are different, the method in the code is of no physical significance, hence could not really meet the balance of internal force vectors. In this paper, we demonstrates that the theoretical system of the method in the code is not rigorous, which might give rise to wrong result when the values of φ_i are not equal. In view of this, we optimized the method by replacing ΔQ_i with shear force Q_i between blocks, and proposed an iterative solution based on single variable η. Through typical examples and engineering applications, we further verified that the imprecision of the standard method may lead to unacceptable calculation errors; and at the same time, we proved the accuracy and practicability of the proposed method in this paper.