The invention discloses a motor and speed reducer connecting structure. A motor comprises a shell, a front end cover and a motor output shaft; a speed reducer comprises a housing, a driving gear shaft and a driven gear shaft; the driving gear shaft comprises a driving gear part, a first front shaft part and a first back shaft part; the driven gear shaft comprises a driven gear part, a second front shaft part and a second back shaft part; the housing covers the front end cover; the motor output shaft is provided with a cavity; a pair of pin holes is formed in the side wall of the cavity; the first back shaft part is inserted into the cavity; a penetrating slot penetrating through the side wall is formed in the back end surface of the first back shaft part; a pin penetrates through the pin holes and the penetrating slot; a first front bearing and a first back bearing are arranged on the first front shaft part and the first back shaft part; the first back shaft part is provided with a shaft shoulder; the back end surface of the first back bearing presses against the shaft shoulder; the front end surface of the first back bearing presses against a retainer ring fixed on the front end cover; the second back shaft part of the driven gear shaft and the driven gear part are arranged on an axis in a sleeving manner. The motor and speed reducer connecting structure is short in axial installation distance, simple and compact in structure and high in transmission efficiency.