The bias caused by differential polarization loss directly affects the performance of four-frequency differential laser gyroscopes. This study proposes an optimization method for the configurable parameters of mirrors, and effectively reduces the differential polarization loss. Based on the Jones matrices, the self-consistent equation is numerically solved, obtaining the influences of the mirror parameters and the non-coplanar folding angle of the four-frequency differential laser gyroscope on the differential polarization loss. The mirror configurations are then improved. In the numerical calculations, the differential polarization loss is most affected by the reflection-induced retardance of the mirror. When the sum of the reflection-induced retardances of four mirrors is 0 and the positive and negative reflection-induced retardances of four mirrors can wipe one another out, the differential polarization loss is 0. This technique will be valuable for reducing the differential polarization loss in engineering applications.