The underwater explosion action within the upstream reservoir area of high dam will produce strong shock waves on concrete dam and affect the stability of dam,even cause the break of dam. Using limit equilibrium method, this paper quantitatively analyzed the force situation of dam under the action of underwater explosion force. A related gravity dam example was given to estimate the ultimate bearing capacity of the dam to resist damage, and then the differential stress calculation method is used to optimize the calculation method of stress in front of the dam. The result showed hat the proposed method can roughly reflect the force of explosion witch accurately effect on concrete dam in the front of dam.