The invention discloses a vertical shaft gear box with simple structure. The vertical shaft gear box comprises a spindle box. The gear box is arranged on the spindle box. A sliding transmission gear shaft is arranged in the gear box in a relatively rotating way. The sliding transmission gear shaft is provided with a sliding gear in a sliding way. A shaft location pit is arranged on the sliding transmission gear shaft. The position of the shaft location pit on the sliding transmission gear shaft corresponds to a first power output gear and a second power output gear. The sliding gear is provided with a wheel inner positioning hole which is internally provided with a wheel inner positioning spring. One end of the wheel inner positioning spring is provided with a wheel inner positioning ballwhich is arranged in the shaft positioning pit. The other end of the wheel inner positioning spring is provided with a wheel inner positioning plug which is connected with the sliding gear in a threading way. The vertical shaft gear box is a vertical shaft speed change mechanism, a power output shaft, a sliding transmission gear shaft and a motor input shaft of the vertical shaft gear box are mutually parallel and are vertical to the horizontal plane. The off-gear problem of the sliding gear due to the dead weight is solved by the twice positioning in one operation.