The problems of laser beam propagation in strong turbulence and the propagation along a slant path are hot and hard points in the subject of light propagation in random media. A model of the second-order moment of laser beam in any turbulence condition was elected. First the expression of the mutual coherent function was derived by using the extended Huygens-Fresnel principle. The model with three parameters for the second-order moment was created and the recursion formulas for the parameters were derived. The iteration formula of the parameters were used to calculated the effective beam radius and the length of coherence in the strong turbulence and in turbulence along a slant path. Compared with the results obtained by directly using the extended Huygens-Fresnel principle in strong turbulence, the effective beam radius obtained by the iteration formula is smaller and the corresponding coherence length is larger. The iteration method also shows that the characteristics of the uplink and downlink laser beam along a slant path are different.