Abstract: For direct detection,long-range CO2 differential absorption lidar (DIAL) system,two factors which have effects on measurement sensitivity:atmospheric turbulence and laser speckle are analyzed.Numerical calculation results of the dependent relationship between measurement sensitivity and atmospheric turbulence and laser speckle are obtained.These results show that when the height is smaller than 350m,measurement sensitivity can be efficiently improved by increasing the height of equipment above ground;when the height is bigger than 350m,measurement sensitivity can be efficiently improved by changing system parameters.