The important parts of a screw compressor are the two rotors. At present, generally use a CMM to check the form and other parameters of the rotors to ensure its function. Among the all geometrical parameters of a rotor, the form of traverse section profile is the most important one. In the process of measurement, the key technology problem is the probe radius correction because of the complexity of 3D helical surface. When the nominal values of the rotors are unknown, the radius correction is more difficult than that is known. This paper presents a method to evaluate the precision of measuring screw rotors by simulation. Suppose a referenced helical surface, first select the probe ball radius and sampling density, calculate the coordinate values of the probe ball centers along a fixed plane, then calculate the values of measured contour by radius correction, finally compare the reconstructed contour with the referenced surface to evaluate the selected measuring parameters such as probe ball radius, sampling density, the algorithm of radius correction and so on. This simulation process is necessary for actually measuring a screw rotor.