Abstract: The problems existing in the conventional vacuum pumping system of wet cooling 660 MW units are analyzed.According to the characteristics of the vacuum pumping system of double back pressure condenser,a modification scheme of adding one set of Roots-water ring vacuum pump set on the basis of the existing system is proposed.The constitution and operation control strategy of the vacuum pumping system of Roots-water ring vacuum pump set are described.Engineering practice has proved that the Roots water ring vacuum pump set can effectively reduce the energy consumption of the vacuum system,and can effectively prevent the cavitation phenomenon,and improve the operation reliability of the vacuum system.