The mathematical expression of the relationship between degree of polarization(DOP)and differential group delay(DGD)for Gauss pulse is derived.Based on the expression,we get the conclusions that the relationship between DOP and DGD is decided by the width of optical spectrum and the splitting ratio.The theoretical analyses show that besides the splitting ratio,only the parameters which are related with the width of optical spectrum can affect the relationship of DOP-DGD,such as modulation formats,modulation chirp,and the trnasform-limit pulse width;and the other parameters can not affect the relationshi of DOP-DGD,such as fiber dispersion,line chirp,and the actual pulse width.We gave the exepriments with 10 Gbit/s RZ pseudo random sequence to demonstrate the validity of theoretical analyses.