Based on the theory of Gaussian-Schell model source and Collins diffraction integral formula, the sufficient condition for invariant polarization of stochastic electromagnetic beams through astigmatic optical systems is derived. Both analytical and simulated results indicate that, when the astigmatic coefficient of optical systems is large enough and it satisfies certain restrictive condition with source parameters, degree of polarization of beams will keep invariant in propagation. As a result, the propagating degree of polarization is as the same as its initial value in the source plane. This sufficient condition is sole, and it is independent of optical systems′ elements and propagation distance, etc. The results may provide potential merits and applications in polarization imaging and detecting in optical systems.