In order to reduce the error of range differential absorption lidar for detecting atmospheric CO₂,theory of detection accuracy is analyzed and the error of differential absorption detection system is simulated.Based on 1.6 μm optical fiber laser, the accuracy of coherent system is calculated. Results show that the signal to noise ratio of echo is higher and error of gas concentration is lower when the differential absorption cross section gets bigger and the spatial resolution gets worse. If the differential optical depth of CO₂ is equal to 0.55, the percent variation of error is the least and the accuracy is the best. With the height increasing, the accuracy of 1.6 μm optical fiber laser coherent detection system is decreasing. When the height is lower than 1 km, the atmospheric CO₂ variation of 34 ppm can be detected.