The phase shifter is a key element in phase shifting interference metrology. Its calibration is an important process that cannot be neglected. The accuracy of calibration has a great influence on the measuring precision. It determines the success or failure of a phase shifting technique. A calibration method of a phase shifter is presented in this paper. It owns the features of high precision and has no special requirement for the linearity of the phase shifter. The calibration is performed with the same optical set up as it is actually used. The errors owing to the different optical paths for calibration and for actual use are thus deleted.