Aiming at the effect of multiple scattering under low visibility weather conditions, an improved method is proposed for inverting slant-range visibility based on the multiple scattering lidar equation. First, the semi-analytical Monte Carlo method is used to calculate the ratio m of multiple scattering to single scattering based on the backscattering peak fitting the scattering phase function. Second, using the Collis method, the visibility level value is obtained, and the corresponding m value is substituted into the multiple scattering lidar equation. Then, the Fernald method is used to invert the extinction coefficient and solve the slant-range visibility. Experiments were performed on the echo signals of haze and heavy rain with different visibility levels detected by lidar, and the slant-range visibility results obtained using the fitting method in this paper and the commonly used Henyey-Greenstein (HG) function method were compared. The results show that the slant-range visibility difference is 10.9% under haze conditions and 5.6% under heavy rain conditions. The proposed method improves slant-range visibility accuracy inverted by the backscattering lidar.