The optical model of image degradation in homogeneous path is widely used in restoration of degraded image in atmosphere. However, when observing in slant path, the illumination and optical characteristic are inhomogeneous in the path of sight. This also occurs even though in horizontal path. The optical model of image degradation in inhomogeneous path is founded based on radiative transfer theory of the parallel plane atmosphere, and the simulation analysis on the model under typical atmospheric and observing conditions is performed. From the results, the degradation model in inhomogeneous path is reduced to the model in homogeneous path when observing near horizontally. While the observing distance is short, the two models are consistent in expression form. And compared with longer wavelengths, the conclusion is better satisfied at shorter wavelengths. The effect of different azimuthal angle differences on degradation model lies only in the difference of background skylight in the position of observer. In addition, the degradation model depends on the near-ground horizontal visibility.