Piston oil-free dry-type mechanical vacuum pump technology is not complicated, easy to manufacture. The shape of the pump was flat square, is composed of four stepped piston and four stepped cylinder. The shaft directly connected with the motor drives four pistons through four connecting rods. The back space of the piston is exhausted by the secondary piston to reduce leakage and reduce the ultimate pressure. Each cylinder has intake and exhaust valves. In order to increase the pumping speed, the two pistons are connected in parallel. In order to facilitate the pump to start at atmospheric pressure, an auxiliary exhaust valve is provided. When the exhaust pressure of the piston is higher than the atmospheric pressure, A part of the gas is discharged into the atmosphere through the valve. When the pressure of the exhaust gas is lower than the atmospheric pressure, the discharged gas sequentially enters the third and the fourth cylinder is compressed and discharged to the outside of the pump. Pump compression ratio up to 105, the pump's ultimate pressure up to 1.3Pa. The inner surface of the pump cylinder is lined with a polymeric material to reduce friction and wear. This pump low power consumption, cooling conditions are good, do not need cooling water.