In the polarization division multiplexing system, the communication capacity can be doubled, depending on the transimission of two othogonal polarized beams as different channels in one optical fiber. These channels need to be seperated in the receiver end. Besides the polarization mode dispersion (PMD), polarization dependent loss (PDL) also affects the stability of system. On basis of the principles of PDL, the numerical relationship between the included angles of two orthogonal signal channels and PDL is derived for polarization division multiplexing system, and the effects of PDL to the system are also analyzed. According to the theorical model, the corresponding experimental structure is built to verify the theory through one axis attenuation in the optical fiber and the test curve description. The experiment shows that the crosstalk can be controlled below -10 dB when differential group delay (DGD) is less than 10 ps. Up to 140 ps, system cannot work any more with the same signal power of two channels. The research is useful to the stability of system.