Abstract: A gear shifting mechanism includes a spindle, a drum shaft, a fork guide, a reverse shaft, and a main shaft, wherein the spindle is provided with a first gear, the drum shaft is provided with a drum, a second gear is mounted on each side of the drum, the first gear of the spindle is meshed with the second gear of said drum, said fork guide is provided with a fork having an end engaged with a sliding rail of said drum and another end engaged with a drum gear shift, and said drum gear shift is provided with a forward gear and a rear gear at two sides thereof, characterized in that said drum is sleeved over said drum shaft and two compression springs urge against two sides of the drum to keep the drum in place, thereby enabling the drum to slide on the drum shaft.