A novel method based on the scanning spectrum system is proposed to calibrate the compensator. One multiple-order half wave plate is added to be used as a benchmark of the retardation. Any retardation between 0 and λ can be measured in high precision. The method can avoid errors caused by the light intensity measuring method,and moreover it can measure the zero-order retardation which the scanning spectrum method hardly accomplished. In the experiment,a compensator is calibrated based on a multiple-order wave plate at 633 nm. It is demonstrated that the retardation measuring precision is 0.45 nm in 0 and λ while the maximum error is less than 3.2 nm between the two positions.