Abstract: The power calculation model of a variable pitch screw vacuum pump in the process of suction, exhaust, compression, and backflow was established, and the shaft power calculation model was deduced.The influence of rotor geometry parameters, circumferential clearance, rotational speed, and inlet pressure on the power was analyzed using the transient steady flow simulation method, and the results were compared with those of the built model.The results show that, within the design range of the studied sample parameters, there was a maximum value of the shaft power in the process of changing the addendum circle and dedendum circle diameter from large to small, and the lead from short to long at the same time; the shaft power increased with the circumferential clearance and rotational speed; when the inlet pressure was 500~1000 Pa, the shaft power increased with the inlet pressure.Finally, it was verified by experiments that the trend of experimental data, theoretical data, and simulation results were consistent, which demonstrates that the proposed power calculation model and simulation method are feasible.