The achromatic characteristic of polarization modulation module- double combo wedges modulator was studied, in order to improve the sampling accuracy of spectral polarization data and image in spatial modulating spectropolarimetry. According to the achromatic principle and the structure of the double combo wedges , the relationship between the first derivative of the phase retardance of the double combo wedges modulator and the birefringence and the wedge angle was deduced, which the double combo wedges modulator was composed by different birefringent crystal materials. The Matlab program was used to find the optimal combination. Based on the theory, the experimental contrast was carried on with KDP-Quartz and Quartz-Quartz combination .The experimental results show that the data of the first-order bright stripe of the KDP-Quartz combination is smaller than that of the Quartz-Quartz combination. The achromatic characteristic of KDP-Quartz combination is better than that of Quartz-Quartz combination, which is consistent with the theoretical analysis.