By the extended Huygens-Fresnel principle,the change of polarization state of partially coherent light propagated in random media was studied.As an example,partially polarized Gaussian-Schell model(PGSM) beam was analyzed in detail.The results show that degree of polarization of a PGSM beam will oscillate because of the influence of random turbulence.The spatial coherence of a PGSM source has a strong influence on the oscillation of degree of polarization.