A new type of method for estimating extinction efficient boundary values with least square fitting for inversion of slant angle lidar measurement is reported. In cloudless weather, the boundary values of extinction is calculated by fitting lidar return signal. For cloudy weather, firstly, the measured region is divided into cloudy and cloudless section. Then it is assumed that the cloudless region is homogenous circumstance. The boundary values of extinction is calculated by fitting lidar return signal except the cloudy region. Finally, the atmospheric extinction coefficients are inversed with Klett's inversion algorithm and the boundary values. The simulation results are close to the real atmospheric conditions.