Using a formula for the ray deflection angle. We obtained a clear and detailed interpretation for the ring pattern of laser-induced thermal self-defocusing. The far-field intensity distribution at different input powers and different positions of beam waist are discussed. The threshold power for the ring pattern and a formula for the number of rings are obtained. All of the theoretical conclusions have been confirmed by our experiments. In addition, a fine structure in the ring pattern resulting from the diffraction effect is also described.