Abstract： To improve the circular test accuracy of double ball bar (DBB), the mechanism of DBB setup errors was studied and an identification and separation model was proposed. In the first place, based on the analysis of the geometric relationship of an arbitrary position in the process of measurement, the mathematical relationship between DBB measurement and setup errors was established. The distortion rule of measured values along with setup errors was obtained in mathematical simulation, and a new identification and separation method of setup errors was put forward. The simulation results prove that the proposed method can identify setup errors with high accuracy. The noncontact laser measurement results show that the proposed method can effectively identify DBB setup errors and significantly improve the accuracy of circular tests.