Based on a multiwave mixing model, we explain the symmetrical and broad light-induced scattering effect in LiNbO3: Fe crystal sheets. Analytical results are given in the undepleted-pumps approximation. The competition between forward small-angle scattering and climbing lights and its relation to the ratio of the size of the incident-light spot to the crystal thickness are discussed. The intensity angle distribution of the forward small-angle light-induced scattering is studied theoretically and experimentally. We suggest that the selfdefocusing effect of the LiNbO3: Fe crystal plays an important role in the formation of the fanning intensity angle distribution.