The invention discloses a speed changing device of a motor tricycle with gear reversing function, which comprises a main speed changing unit, an auxiliary speed changing unit and a driving device, wherein the auxiliary speed changing unit enables a gear reversing mechanism which realizes the gear reversing function to be arranged in the auxiliary speed changing unit, and the driving device is connected between the auxiliary speed changing unit and the main speed changing unit. Because the auxiliary speed changing unit is arranged, on the basis that original integral space is reasonably used, the speed range of the motor tricycle is greatly enhanced, thereby the economical efficiency of fuel and the whole carrying capacity of the motor tricycle are increased. Meanwhile, because the gear reversing mechanism and a mutual interlocking gear are arranged in the auxiliary speed changing unit, the space of an auxiliary gear box casing is fully used, the manufacturing cost is greatly decreased, the gear reversing demand is satisfied, and the stage changeover reliability is enhanced.