Based on the prevailing problems of the Roots water-ring vaccum pump combination extraction system for steam turbines that the inappropriate configuration of vaccum pumps leads to the increase of condenser vaccum degree, this paper put forward a Roots water-ring pump frequency conversion control extraction system. Based on the mathematical model of vaccum pumps, a simulation experimental platform for the extraction system is established. Aimed at increasing the pumping capacity and reducing energy-consuming, the optimal operation plan is beterminede, the control strategy of the frequency conversion system is developed, the condenser step tests are performed. The results show that the control strategy of the pumping system is reliable and the system runs smoothly. And the frequency conversion system has obvious energy saving effect compared to the fixed speed system.