The present invention provides a mounting structure and a gear motor. The gear motor includes a motor and a gearbox, wherein the mounting structure mounts a part of the gear motor on another part. The mounting structure comprises at least one first hole portion formed on a first part of the gear motor, the first hole portion being a through hole; at least one second hole portion formed on a second part of the gear motor, the second hole portion being a through hole or a blind hole and being disposed coaxially with the first hole portion, and the second hole portion being larger or smaller in diameter than the first hole portion; and at least one expansion element which is inserted through the first hole portion and the second hole portion, wherein the expansion element forms an interference fit with one of the first hole portion and the second hole portion, and forms a transitional fit with the other one of the first hole portion and the second hole portion. The expansion element can be effectively prevented from slipping out during assembly and operation of the gear motor, the difficulty of assembly of a gear motor can be reduced, and the deformation of the expansion element during assembly can be minimized.