The utility model discloses a water inlet self-destruction mechanism of an underwater closed detector, which solves the problems of complex structure and poor reliability of the water inlet mechanismof the existing closed detector. An inner cavity annular step (16) is arranged on the inner side wall of the piston cylinder (5); an annular cavity (7) is formed between the side wall of the inner cavity of the piston cylinder (5) below the annular step (16) of the inner cavity and the piston (3); a grain groove (17) is formed in the lower bottom surface of the piston (3); the grain groove (17) isfilled with a grain (9); a groove opening of the grain groove (17) is provided with a grain baffle (12); an igniter (13) is connected with the grain (9) through a grain baffle (12), an air passage (8) is communicated between a grain groove (17) and the annular cavity (7), a shear pin insertion annular groove (18) is formed in the side wall of the piston (3), and the end of a shear pin (4) is inserted into the shear pin insertion annular groove (18); manufacturing and maintenance simplicity.