As a new tool to detect atmosphere, lidar can derive the value of visibility by directly collecting the scatterd laser from the atmosphere. It makes lidar become a main instrument for measuring visibility. The schematic diagram and key parameters from the mobile Raman-Mie scattering lidar (RML) are given, and the slope method for getting visibility from data collected by lidar is presented.A practical experiment is conducted in Mianyang and the result is compared with that by Belfort visibility sensor.The results show that RML lidar has high reliability and is precise. The error of the measurement is less than 20%.