To identify the effect of contraction joint state on the anti-knock performance of concrete gravity dam, numerical simulations based on fluid-solid interaction(FSI)method are conducted, which simulates the formation and propagation of shock wave due to shallow underwater explosion, and its interaction with free surface and dam structure. Considering that the shock wave propagation is influenced by the opening states of the contraction joints in the dam, the impact of the latter on the anti-knock performance of the dam is hence investigated in three aspects, including dynamic response, gradual damage failure process and the accumulation of damage dissipation energy. The numerical results show that the anti-knock performance of the dam is significantly influenced by the opening area of the contraction joint, a larger opening area appears to yield enhanced dynamic response, wider damage area and larger accumulation of damage dissipation energy in the dam, thereby weakening the anti-knock performance of the dam.