At present, the height profile of PM2.5 mass concentration is difficult to detect directly. However, a certain relationship can be used to transfer the height profile of aerosol backscattering coefficient detected by lidar to the height profile of PM2.5 mass concentration. The hygroscopic factor of aerosol is a key parameter in the process of transferring the profile of aerosol backscattering coefficient to that of PM2.5 mass concentration. The near ground height distribution profile of aerosol backscattering coefficient can be detected by the lidar system, while the PM2.5 mass concentration and atmospheric relative humidity are measured by PM2.5 monitor. The two instruments are placed at the same location and probed simultaneously to obtain the aerosol backscattering coefficient, PM2.5 mass concentration and atmospheric relative humidity at the same time. Under the guidance of the theory, the expression hygroscopic factor is calculated by fitting the related experimental data. By fitting the detection data aerosol backscattering coefficient, PM2.5 mass concentration and atmospheric relative humidity in Hefei area from May 2016 to October 2017, the change law of the hygroscopic factor between PM2.5 mass concentration and aerosol backscattering coefficient is obtained.