Based on Fernald method, the function relation between the set value of the lidar ratio and the optical depth of aerosols within the lidar detection range is studied. In view of the continuity of the function, the new algorithm for retrieving low layer arosol slidar ratio is proposed, in which the dichotomy is used and the optical depth of aerosols is the convergent criterion. In late July 2014, the sun photometer and the self-developed Raman-Mie lidar are used to detect the atmosphere in Xi′an. Then the proposed algorithm is used to retrieve the lidar ratio and the extinction coefficient. The experimental results show that, the lidar ratio of aerosols in Xi'an is relatively stable during the observation period, the value is 44 or 45. Due to the impact of atmospheric humidity the lidar ratio is larger in the day of rain, the value is 51. The fine retrieving of aerosols lidar ratio is accomplished by iteration, and in the iteration only the Mie scattering signal of lidar and the detection data of sun photometer are used. This is very important for studying the fine retrieve algorithm of aerosol′s optical characteristics.