Abstract: A theoretical model for describing the pumping performance of compound molecular pump’s drag stage was established. According to the theoretical model， the relationship between the depth of drag stage’s spiral groove and compression ratio was studied under different spiral angles， rotor-stator clearances， rotor speeds and gas flows. The results showed that compression ratio firstly increases and then decreases with the decrease of spiral groove’s depth. There exists the optimal groove depth， which result in the maximum compression ratio of drag stage. The spiral angles have little effect on the optimal groove depth which decreases with the decrease of the rotor-stator clearances， and increases with the increase of the rotor speed. The optimal groove depth is smaller when the gas flow is viscous.

Key words: compound molecular pump, drag stage, theoretical model, compression ratio, optimal groove depth