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Based on the Monte Carlo method, a model of ocean - atmosphere coupled polarized radiative transfer under the spherical condition is constructed. The model stratifies the spherical atmosphere in parallel with the sea surface as the underlying surface. An absolute reference coordinate system is established with the center of the earth as the origin, and the geometry of the solar incidence, detector observation, as well as the position，the motion direction and motion state of the simulated photon are projected onto this coordinate system. In order to improve the simulation efficiency, the backward Monte Carlo method is adopted, which greatly improves the contribution efficiency of photons in the multi-layer spherical atmosphere motion. The scattering and absorption of photons by atmospheric molecules and the reflection of sea surface on photons are introduced in detail. The Monte Carlo model constructed in this paper is used to simulate the upper radiation of the spherical and plane-parallel atmosphere, respectively. At the same time, we use the two models to verify each other. We find that the results obtained by the spherical model are quite different from those obtained by the aspheric model in the case of large solar zenith angle or large observing zenith angle. The model constructed in this paper lays a foundation for establishing the atmospheric correction algorithm under the great solar zenith angle.
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