The invention relates to the technical field of cooling of motorcycle engines and discloses a cooling device for a piston of a motorcycle engine. The cooling device comprises a crank case, a crank rodpiece arranged in the crank case, a piston cylinder and the piston arranged in the piston cylinder; the piston cylinder is arranged above the crank case; the piston reciprocates up and down to drivethe crank rod piece in the crank case below to rotate; a cavity is formed in the lower part of the piston, so that heat-dissipation cooling is accelerated; the crank rod piece extends into the cavityand is connected with the piston; besides, a cooling oil channel is formed in the wall of the crank case, so that the cooling oil channel does not occupy the space of a crank case body and is protected; an oil spray opening of the cooling oil channel is formed below the piston, and the cooling oil channel is connected with a hydraulic pump, so that cooling oil is sprayed onto the piston to cool the piston and lubricate the crank rod piece below; an oil inlet of the hydraulic pump is connected with an oil tank; and the oil tank communicates with the crank case to recover the cooling oil timely.