Based on the theory of partial coherence in spacefrequency domain, the change of polarization state of a partial coherent light beam passing through a Fourier trans form system is studied. As an example, Gaussian Schellmodel (GSM) beam is analyzed in detail. An explicit expression is obtained for the degree of polarization at the output plane of Fourier transform system. The results show that the spatial coherence of incident field has a strong influence on the polarization state of ouput beam. The polazrization degree of output beam depends on the ratial between the spatial correlation lengths of two components of incident field. Radial-distributions of the polarization degree of output beam are different for defferent Fourier transform system.