A new method is presented to calibrate lidar constant. Atmospheric aerosol optical depths(AOD)were measured by solar radiometer and lidars were used to acquire backscattering signals from aerosol free-loading regions 35 to 40 km altitude. Atmospheric model provides molecular scattering coefficient. Lidar constant can be calculated according to lidar equation. After lidar constant was calibrated using solar radiometer,aerosol optical depth can be determined with lidar return signals from 35 to 40 km altitude according to lidar equation. A good agreement between lidar measurements and solar radiometer measurements shows that our method for AOD measured by lidar is feasible. This method can be used to measure AOD on daytime and nighttime.