Based on the study of the new requirements of the precision and ultraprecision machine technique for the error separation technique of straightness measurement, this paper analyzes quantificationally the error introduced into the time domain and frequency domain EST results by the misalignment of sensors, the neglect of yaw angle in sequential two-point method and the difference of drifting of these sensors. This paper also describes quantificationally the magnifying ability for measurement errors of the factors W(k) in frequency domain EST methods. Some measures were taken to reduce this influence. The conclusion that sequential two-point method is the best fit method for straightness measurement was arrived at the end of this paper.