The invention relates to the field of printing equipment, and discloses an installing structure of a speed reducer. The installing structure is used for fixing the speed reducer to a working machine and comprises an installing plate, one end of the installing plate is connected with the speed reducer, the other end of the installing plate is connected with the working machine, and the speed reducer is rotationally connected with the working machine. The speed reducer is fixedly connected to the working machine through the installing structure of the speed reducer, and therefore the circumferential movement of the whole speed reducer is limited, the coaxiality requirement for matched connection of an output shaft of the speed reducer and a driving shaft of the working machine in the assembling process can be reduced, the assembling difficulty can be reduced, the assembling efficiency can be improved, and the speed reducer can be effectively prevented from being damaged.