In this paper

a kind of componnd rotor profile of 3 lobes Roots blower/vaccum pump was put forward. There is a section of straight line on the working part of the rotor profile. By applying the Basic law of conjugate gear tooth action

the meshing of all the composing sections of the rotor profile was analyzed

the meshing equations of the straight line section and its conjugate profile were built. The continuity of the meshing of the rotor and the phenomenon of multipoint meshing in the rotor profile were studied. The rotor profile had been used succesfully in production.