A drive mechanism, applied to an automatic door operator, the drive mechanism including a planetary reducer driven by an electric motor, and a drive shaft driven by the planetary reducer, wherein: the planetary reducer includes a housing and a planetary gear train mounted in the housing; the planetary gear train includes a final-stage planet carrier configured to output torque, and a center of the final-stage planet carrier is provided with a torque output hole; and a first end of the drive shaft is detachably inserted in the torque output hole and is non-rotatably connected to the torque output hole, and a second end of the drive shaft extends out of the housing, wherein: the planetary gear train further includes a first-stage sun gear, a first-stage planet gear, a first-stage planet carrier, a second-stage sun gear and a second-stage planet gear; an output shaft of the electric motor is connected to the first-stage sun gear, and the first-stage planet gear is mounted on the first-stage planet carrier and engaged with the first-stage sun gear; and the second-stage sun gear and the first-stage planet carrier are attached together or integrally formed, and the second-stage sun gear is engaged with the second-stage planet gear mounted on the final-stage planet carrier, characterized in that a center of the first-stage planet carrier is provided with a through hole, a center of a first end face of the drive shaft is provided with a pin mounting hole, and a pin is mounted in the through hole and the pin mounting hole. the planetary reducer includes a housing and a planetary gear train mounted in the housing; the planetary gear train includes a final-stage planet carrier configured to output torque, and a center of the final-stage planet carrier is provided with a torque output hole; and a first end of the drive shaft is detachably inserted in the torque output hole and is non-rotatably connected to the torque output hole, and a second end of the drive shaft extends out of the housing, wherein: the planetary gear train further includes a first-stage sun gear, a first-stage planet gear, a first-stage planet carrier, a second-stage sun gear and a second-stage planet gear; an output shaft of the electric motor is connected to the first-stage sun gear, and the first-stage planet gear is mounted on the first-stage planet carrier and engaged with the first-stage sun gear; and the second-stage sun gear and the first-stage planet carrier are attached together or integrally formed, and the second-stage sun gear is engaged with the second-stage planet gear mounted on the final-stage planet carrier, characterized in that a center of the first-stage planet carrier is provided with a through hole, a center of a first end face of the drive shaft is provided with a pin mounting hole, and a pin is mounted in the through hole and the pin mounting hole. the planetary gear train further includes a first-stage sun gear, a first-stage planet gear, a first-stage planet carrier, a second-stage sun gear and a second-stage planet gear; an output shaft of the electric motor is connected to the first-stage sun gear, and the first-stage planet gear is mounted on the first-stage planet carrier and engaged with the first-stage sun gear; and the second-stage sun gear and the first-stage planet carrier are attached together or integrally formed, and the second-stage sun gear is engaged with the second-stage planet gear mounted on the final-stage planet carrier, characterized in that a center of the first-stage planet carrier is provided with a through hole, a center of a first end face of the drive shaft is provided with a pin mounting hole, and a pin is mounted in the through hole and the pin mounting hole.