In order to improve the calculation accuracy of slant visibility, a method based on Fernald-PSO is proposed to determine the aerosol lidar ratio. Firstly, the equation of aerosol lidar ratio and aerosol extinction coefficient boundary value is constructed under the condition that atmospheric extinction coefficients are equal at the bottom and the top of the uniform layer of the atmosphere. The second equation is constructed under the condition that aerosol optical depth derived from the lidar signal is equal to that obtained in AERONET. Finally, Fernald-PSO is used to solve equations, and slant visibility is obtained using the inverted aerosol extinction coefficient. The proposed method is verified by lidar and AERONET data, and the effects of aerosol lidar ratio on the slant visibility are analyzed. The results show that for the same signal, the larger difference between the assumed and calculated values of aerosol lidar ratio, the greater relative errors’ absolute value of aerosol extinction coefficient and slant visibility. The maximal errors are 18.93% and 19.90%, respectively.