Based on the CCD side scattering lidar (Clidar) and Mie scattering theory, a model of the relationship between the side scattering light intensity distribution and the PM2.5 concentration is established, and the measurement error is analyzed. In the study, the system setup of Clidar PM2.5 concentration measurement, which uses a laser at 532 nm as a light source and a CCD as a receiver, is constructed. The obtained side scattering echo signal is recorded to compare with the PM2.5 concentration measured by the BAM-1020 particle monitor, and hierarchical model expressions under high and low gains are built. When we compare the predicted values of hierarchical model PM2.5 concentration with the measured values, the average error, residual variance and comprehensive deviation rate statistics of each model are obtained. The results show that the inversion accuracy of the high gain model is better than that of the low gain model when the PM2.5 concentration is in the range of 0-70 μg·m -3. In the high gain model, the inversion accuracy of S(20) model is the highest.