Abstract: In order to insure the accuracy in linear polarimetry, a fou-beam method was presented to calibrate the retardance of quarter wave plate. The method was based on spatial modulating optical structure, which was composed of quarter wave plate, combo birefringent wedge and polarizer. Variety of polarization measurement and polarization measuring accuracy was researched by simulation approach, when degree of circular polarization of target was changed. It was indicated that the measuring accuracy of Qi, Ui and degree of linear polarization were better than 10-3 with retardance calibration, when degree of linear and circular polarization is randomize between 0.1 to 0.2 and 0 to 0.2 respectively. Accuracy of polarization parameter measuring, except Qi, is about 14 times higher than those without retardance calibrating process.