An approximation model of multiple-scattering among horizontally adjacent fields is proposed as a means of improving the three-dimensional (3D) radiative transfer calculation for clouds in remote-sensing applications. Horizontal radiative-exchange equations are established after analyzing the mechanism of radiation flux density variation among horizontally adjacent cloud units. By introducing the solar-compensation function, the influence of the solar incidence angle upon radiative transfer is corrected. The experiment is conducted using I3RC Phase Ⅱ cumulus (Cu), stratocumulus (Sc) and altocumulus (Ac) data generated by the multi-scale superposition fractal algorithm based on actual observation and correction. The experimental results show that compared with the independent pixel approximation (IPA) and combined strict single-scattering and Eddington multiple-scattering (SSEddMS) models, the mean relative error of the upwelling source function (USF) calculated using the proposed model is better than 13% when the solar zenith angle is in the range of 0°-60°. The accuracy of the proposed model is improved by even more than 15% under low solar zenith angles. The accuracy of the pixel-level-radiance calculation of the proposed model falls within 5% under different observational conditions. Furthermore, it can be applied to 3D clouds with different optical thicknesses and horizontal non-uniformities. This has obvious advantages for stability, applicability, and accuracy.