Based on the extended Huygens-Fresnel principle and the method of Stokes parameters, the model of degree of polarization (DoP) for partially coherent flat-topped electromagnetic beam propagation in slant turbulent atmosphere is studied. Not only the DoP in the same direction and the DoP in the cross-direction are considered, but also the effects of the beam order, zenith angle, frequency, receiving height and other factors on the DoP are discussed. The numerical results show that the effect of zenith angle on the DoP can be reduced by increasing the receiving height when laser transmits in slant turbulent atmosphere. The DoP decreases with the increase of beam order and the increase of frequency near the center angular frequency. Unlike the DoP only considering the same polarization，the change of DoP is unbounded with the increase of propagation distance when the cross polarization is considered.