Aiming at the accuracy of roundness error separation in precision machining, a new practical method of roundness error separation based on geometric dynamic model is proposed. Based on the synthesized of revolution and rotation of the rotor axis, the geometric dynamic model which represents the motion trajectory of the cross section of axisymmetric body is built. So the roundness error is extracted from dynamic model with the error separate technique of three-point method. In addition, the effect factors of separation accuracy, such as harmonic suppression, interference error propagation, angular error, are taken into account. The simulation and test results show that the dynamic geometric model is effective for roundness error separation, and can improve the accuracy of separation and suppress the effect of errors in workpiece machining.