Within the framework of the soft particle approximation (SA), the small scattering angle representation for the Mie amplitude functions is constructed. The general analytical criterion of transition from SA of any optical characteristic to its anomalous diffraction approximation (ADA) is formulated. The ADA for the small-angle scattering phase function is determined and its applicability domain is investigated. The ADA and the Kirchoff approximation are shown to have the same asymptotic behavior.