Based on Fernald’s and Klett’s method, a new expression of the atmospheric extinction coefficient boundary value was presented. This expression was a formula that has two more terms than Collis’ method. The absolute value of the two terms may account for 78.2% of the term in Collis’ method and have opposite sign. Taking the two terms into consideration, aerosol optical depth (AOD) derived by lidar is closer to the observed one by sun-photometer. With the change of reference height, the derived AOD ranging from 0.20 to 0.25 and the variance is about 0.003, showing that this approach is independent from reference height and relatively stable. 424 clear sky data were selected in the inversion. AOD derived by lidar is 7.4% bigger than the observed one. The correlation coefficient is 0.932, the relative error is 10.9%, the absolute error is 0.03 and the variance is 0.02. The approach has a better performance when AOD is less than 0.45.