A foam pump, comprising a pump body (1), a big cover (2), a big piston (3), a small piston (4), a piston rod (5), a head cap (6) and a foam screen (7), featured by that: said pump body (1) is at the upper part, said big cover (2) is at the lower part, said pump body (1) being integrated with said big cover (2), said head cap (6) being integrated with the lower big piston (3); a sealing ring (9) of elastic materials being provided between said big piston (3) and the inner wall of said pump body (1) for tight engagement, which forms an one-way valve structure, which makes the pump body and the big cover integrated, thus the air cavity can be enlarged, the working pressure of the foam pump can be improved with the big cover of the same size, and the foaming effect of the foam pump can be enhanced; said head cap being integrated with said big piston, a sealing ring is provided between said big piston and the inside wall of the pump body, which simplifies the structure of the big piston and makes the internal structure of the foam pump more compact.