The invention discloses a three-stage gearbox based on a stepped shaft. The three-stage gearbox based on the stepped shaft comprises the stepped shaft, a box body, an input shaft and an output shaft.The output shaft is provided with a first belt wheel, the inner wall of the box body is provided with a support, and the support is internally provided with a second belt wheel which corresponds to the first belt wheel. The tail end of the input shaft is inwards concaved to be provided with a guide hole. The guide hole is internally and movably provided with an expansion link. The stepped shaft penetrates a belt and is concentrically connected with the expansion link. The stepped shaft comprises a first cylinder, a second cylinder and a third cylinder which are sequentially and concentricallyarranged. The top end of a lifting device is provided with a U-shaped fork, and the top of the U-shaped fork is provided with a pressing wheel located above the belt. According to the three-stage gearbox based on the stepped shaft, by means of progressive increase of the diameter of the first cylinder, the diameter of the second cylinder and the diameter of the third cylinder, progressive increaseof a rotating speed of the output shaft is achieved, operation is convenient, and no impact or shake is caused during speed changing.