Abstract: The design of the rotor profile is an important contents of the research for the screw vacuum pump. Cycloidal rotor profile is a commonly used profile, the part located outside the pitch circle is the outer cycloid, and the part inside the pitch circle is the internal cycloid. By selecting the appropriate tooth top circle radius and pitch radius, the rotor profile outer cycloid and internal cycloid equations, as well as the tooth top circle and root circle equations, were analyzed and established. Then a single-head symmetrical cycloidal screw pump rotor profile and rotor is designed. On this basis, the relationship between rotor volume utilization and related parameters is analyzed for the design of cycloidal roots pumps.