A novel method of Mie scattering lidar to determine the lidar ratio in cirrus clouds with the multiple scattering is proposed.Firstly,according to the multiple scatter ing lidar equations,the nonlinear equations with cirrus lidar ratio and cirrus top extinction coefficien t as variables are constructed.Secondly,by using the particle swarm optimization algorith m to solve the equations,the extinction coefficient of the cirrus′s top and th e lidar ratio of cirrus clouds are acquired.Then,through the PLATT′s multiple scattering equation and the ratio of multiple scattering to single scattering given by Qiu Jinhuan,the lidar ratio is obtained.The experiment is carried out based on the proposed method,usi ng the real return signals of the ground-based lidar.It′s turned out that the prosed technique has the advantages of high accuracy and the fast c onvergence of iteration.