The utility model relates to the technical field of mounting tools, and discloses a piston mounting device, which comprises a mounting body and a push rod. The mounting body is a structure with a through hole, the diameter of the through hole is equal to the inside diameter of a cylinder body of a piston to be mounted, and the force application end of the push rod acts on the top of the piston to be mounted. The piston mounting device reduces the labor intensity of maintenance staff, increases the safety factor of operators, is easy to operate, requires no operational experience, greatly reduces the troubleshooting time, and improves equipment operation ratio, thereby creating considerable economic benefits. The piston mounting device has the advantages of simple structure and high safety, and is easy to manufacture, operate, popularize and apply.