In order to use liquid crystal variable retarder (LCVR) to measure the p hase retardation of light,the calibration method of phase retardation is studied.First,the factors that res ult in the change of phase retardation are analyzed in theory.Then,the retardance for 405nm,532nm,632nm and 641nm l aser is measured by the light intensity method based on the Stokes parameters wh ich changes with the driving voltage.Besides,the change of normalized phase retardation is only related to the driving voltage is verified by experiment.Next,the least square method is presented to measure the initial phase retardation for differen t wavelengths,and the calibration equation of initial phase retardation is calculated when different wavelengt hs of light incident to the LCVR.A 671nm laser is used to demonstrate the equation and the results show that the stardard deviation is 1.4nm between the initial phase retardation calculated by the calibration equation and the rea l value,and the maximal relative error between them is 0.18% under any driving voltage.Finally,compared with other methods of calibrating initial phase retardation,the least square method is accurate and re liable.