The method is proposed to improve the response range of DOP Ellipsoid to PMD. A conclusion is given that response range of DOP ellipsoid shortest axes to PMD can be greatly increased by using the proper bandwidth filter and selecting proper pulse width laser in RZ code optical fiber communication system. The results also show that the bandwidth of the filter should be limited above 50 GHz, while pulse width should be limited in 4~10 ps for 40 Gb/s system. Finally, the experiment result shows that the above proposed method are very effective to increase the response range of DOP ellipsoid on PMD in 40 Gb/s system.