A new method is presented based on a Stokes vector and a Mueller matrix to measure the retardation of a Liquid Crystal Variable Retarder (LCVR) attached with a compensator, a 2 kHz square wave with the adjustable amplitudes 0~10 V is used to control the LCVR and the retardation is measured at six wavelengths in a visible region.The experimental results show that the LCVR has the advantages of low voltage controlling, simple operation, wide adjustment ranges and no mechanical rotating,and the error of measurement is less than 0.6%.The fitting function is established based on the characteristics of the retarders,and the retardations of all wavelengths and all voltages in the working condition of the retarders are obtained by the least-square curve fitting.The standard error of curve fitting is less than 0.02λ.The measurement method, experimental results and the fitting function provide a reference for the polarization device research.