A new simplified radiative transfer model of the soil bidirectional reflectance is developed that approximates multiple-scattering radiance by a four-stream formulation and calculates single-scattering radiance and double-scattering radiance exactly. This model also takes any arbitrary anisotropic sky radiance into consideration. The comparisons with the numerical code based on the discrete-ordinate algorithm (DISORT) indicate that the present analytical model has high accuracy much better than the widely used Hapke model, especially when soil particles scatter very strongly or anisotropically. Evaluation of the inversion accuracy of the present model is also discussed.