During the inversion of atmospheric extinction coefficient, choosing the boundary value of the extinction coefficient is critical. In this study, a new method based on lateral Steffensen-like third-order method is proposed for determining the boundary value of atmospheric aerosol extinction coefficient. To confirm the method reliability, the proposed method is applied to both simulated and actual measured echo signals. Results show that the proposed method has high iterative speed and converges to the boundary value of extinction coefficient after a few iterations, which can invert atmospheric extinction coefficient more accurately.