The invention discloses a mechanical speed adjusting device. The mechanical speed adjusting device comprises a lifting box, a first fixed pulley and a first steel wire rope. The speed adjusting devicecomprises a gearbox with openings formed in the front end and the rear end. A first gear shaft, a second gear shaft and a third gear shaft which are rotationally connected with two side plates of thegearbox are transversely and horizontally connected in the gearbox in sequence from back to front. A first large gear, a winding drum and a mechanical hand brake are sequentially arranged on the first gear shaft from one end to the other end. The first large gear and the winding drum are coaxially and fixedly connected with the first gear shaft. The first steel wire rope is wound on the winding drum, and one end of the winding drum is wound around the first fixed pulley to be connected with the top of the lifting box. A second small gear engaged with the first large gear and a third large gear coaxially and fixedly connected with the second small gear are coaxially fixed to the second gear shaft. A fourth small gear engaged with the third large gear is coaxially fixed to the third gear shaft. A speed adjusting sliding sleeve, a first spring and a fly hammer fixing sleeve are sequentially and coaxially arranged at the position, on one side of the fourth small gear, of the third gear shaft. A multi-gear machine is adopted for automatically adjusting the speed, and operating is safe and stable.