The cross-spectral density matrix of Gaussian Schell-model beams through the ABCD optical system is derived，and the variation of the spectral degree of polarization under this condition is given. A comparison between the cases of Gaussian Schell-model beams through the free space and focal optical system is studied. The study shows that the spectral degree of polarization will tend to the stable value after a long-propagation distance in free space，and the spectral degree of polarization has a sharp fluctuation at the position of focal plane in focal optical system. Both are related to the selection of light source parameters. In particular，the spectral degree of polarization at the position of focal plane equals the stable value of the spectral degree of polarization at the far field.