For long2range direct detection CO2 differential absorption lidar (DIAL) ,the detection performance is re2 searched.Both speckle noise and detection noise are analyzed ,which have important effects on detection perfor2 mance. Calculation results of dependent relationships between detection performance and speckle and detection noise are obtained. These results show that the detection performance is mainly limited by speckle noise in the case of a nearby target ,and it is mainly limited by detection noise for target at long range. If unreasonable system parameters are selected ,the detection performance will be severely limited by speckle noise.