In the process of atmospheric visibility measurement based on the backscattering principle, the selection of atmospheric extinction coefficient boundary value has a great influence on the inversion accuracy. In this paper, a method for determining the boundary value of atmospheric extinction coefficient based on secant method is proposed, and the selection result of boundary value is modified by iterative idea. The validity of the method is verified by theoretical simulation and actual measurement. The results show that this approach has faster convergence and fewer iteration times. The extinction coefficient boundary value after iteration can be used to retrieve the atmospheric extinction coefficient more accurately and realize the high precision measurement of atmospheric visibility. And it has great practical application value.