Abstract: In order to study polarization properties of Gaussian-Schell model beams, the method of cross-spectral density matrix was adopted to study polarization properties of Gaussian-Schell model beams propagating through a focal optical system. The conditions, under which complete polarized and complete non-polarized beams kept the degree of polarization invariable in the focal optical system, were provided. The condition where the degree of polarization distributed homogeneously at propagation plane was also given. The degree of polarization had a sharp fluctuation at focal position in a focal optical system. The degree of polarization at focal position equaled the stable value of the degree of polarization at far field. The study had important theoretical and practical significance in the field of laser transmission.