Screw rotor is the most central parts of dry screw vacuum pump.Compared to the constant pitch screw rotor,variable pitch screw rotor leads to the gas pre-compression in the pump cavity,and it can reduce the temperature of the exhaust port signifcantly under the same compression,and the operation of the pump is more stable and reliable.Under the same screw length,variable pitch screw has a greater theoretical pumping speed or compressor stages.That lead of the screw rotor(pitch)from the pump end to the exhaust end of the suction decreases,and reaches a minimum at the discharge end is the common feature of the existing variable pitch screw rotor.Different trends of the pitch results in different compression ratio distribution of the pump chamber,thus affecting the performance of the vacuum pump.To solve this problem,this article uses single concave variable pitch rotor twin-screw pump as the research object,and a mathematical calculation model which is about to leakage and geometric speed of screw pump is created.And a program is made for calculating the actual speed and ultimate vacuum of the vacuum pump under three different pitch change trend that from the suction to the exhaust end changes from large to small,from small to big,and even changes evenly.