In order to analyze the principle error and the principle error’s regularity and characteristics in the two point error separation method, a part model in the time domain was established to make the error separation. Through the analysis of different influence factors, the principle error’s regularity and characteristics of two point method error separation technique were found. Using two point method error separation technique and roundness measuring instrument to measure the same parts, it was found that the experiment results and the simulation results agreed with the theory analysis. So the conclusions are: when the shaft does not exist eccentric error, facing mounting the two sensors, the error of two point separation results is the biggest; when the shaft exist eccentric error, facing mounting the two sensors, the error of two point separation results is the smallest.