A speed reducer includes an input shaft, an output shaft, and a gear assembly coupled to the input shaft. The gear assembly includes a housing, an external gear, and at least two internal gears. The external gear is located within the housing and inscribes the at least two internal gears. One side of the at least two internal gears is rotatably coupled to the input shaft, and another side of the at least two internal gears is coupled to a connecting column of the output shaft. The at least two internal gears are offset on the input shaft, and the input shaft drives the at least two internal gears to maintain engagement with the external gear. The connecting column maintains transmission of the output shaft and the at least two internal gears.