We improve the degree of polarization(DOP)-based differential group delay(DGD) monitor by applying wideband filter,narrowband filter and asymmetrical filter for same pulse-width return-to-zero(RZ),carrier-suppressed RZ(CSRZ),RZ-differential phase-shift keying(RZ-DPSK),and CSRZ-DPSK signals.Simulated results indicate that the filtering technique can enhance DGD dynamic range and the narrowband filtering is better than any other three filtering techniques to increase the DGD monitor range and is fit for all the formats.The experiment validates the filtering technique is effective for improving DGD dynamic range.