The invention belongs to the technical field of transmission mechanisms, and specifically discloses a planetary gear mechanism. A planetary gear speed-changing device comprises a planetary gear mechanism, wherein a planetary gear device comprises a sun gear, an inner gear ring, and a planetary gear provided with a plurality of fixed shafts, wherein a bevel pinion and the sun gear are fixedly installed on a sun gear shaft separately, the bevel pinion is located at the outer side of the planetary gear mechanism, and a high-speed-gear output shaft and the upper end of the sun gear shaft are coaxially and fixedly connected; a large bevel gear is concentrically fixed to the outer wall of the upper end of the inner gear ring; and the lower end of the inner gear ring is fixedly connected with a connection end cover through a flange, and a high-torque-gear output shaft is fixedly connected to the center of the connection end cover through a flat key. According to the planetary gear mechanism,a planetary gear gearshift mechanism realizes a high-gear and low-gear speed-changing function; and a low-speed-gear mechanism increases a torque, and realizes high-power and high-torque output in theworking conditions of driving a motorcycle to go up a slope by a motor, and the like.