A wire grating beam splitter (WGBS) substrate in a dispersion-compensated polarization Sagnac interferometer (DCPSI) may introduce an additional shear distance in the shear distance generated by the DCPSI, thereby causing poor adaptability of the DCPSI to white light. This work applies a compensation scheme of an optical flat with the same material and thickness as the WGBS and parallel to the WGBS introduced in the other arm of the DCPSI. Theoretically, this method can decrease the additional shear distance approaching 0. The ideal shear distance in the simulation experiment is 5.86 mm, and the shear distance before and after compensation is 5.40 and 5.86 mm, respectively. The theoretical value of the additional shear distance in this experiment is -0.6625 mm, and the average compensation value is 0.66 mm. Overall, experiment and simulation results indicate that the above method can effectively eliminate the additional shear distance.