The invention discloses a piston cooling structure applied to marine diesel engines. A piston is provided with a piston head portion and a piston inner cavity, the piston inner cavity is used for containing a connecting rod small end, an annular cooling cavity is formed in the piston head portion in the circumferential direction of the piston, two oil input and return openings communicated with the annular cooling cavity are formed in the top of the piston inner cavity, and the inlet sides of the two oil input and return openings are integrally connected to form a communicating area. Through the structure, the piston is fully cooled, and is lightened.