In the procedure of retrieving aerosol backscatter coefficient,the boundary value of aerosol backscatter coefficient is always assumed around the troposphere to eliminate the lidar constant. When the range of the lidar detection cannot reach the tropopause,it is very difficult to determine the boundary value. A relationship between boundary value and range square corrected signal is derived from elastic scattering lidar equation. Using this relationship as a criterion,a boundary value can be determined in the lower troposphere with iteration algorithm. A profile of aerosol backscatter coefficient ,which is retrieved with the boundary value determined using a new method introduced in this paper,is compared with the one retrieved with the boundary value assumed around the troposphere. The comparison shows that these two profiles seem to be consistent with each other well. The profile of aerosol backscatter coefficient can be retrieved with the boundary value determined using the new method in the lower troposphere.