Abstract: The characteristics of gain-modulation laser radar images seriously affected by noise in the condition of long distant and weak signal were studied. For the purpose of improving the ranging precision, the processing method with double thresholds was proposed on the basis of the basic principle of the laser radar system. The minimum ranging error equation was obtained from the basic operation principle of gain-modulation laser radar. Considering faulty judgments between target and background with double thresholds processing method, the error probability functions were defined for gain-modulated images and range images. Two buildings at the distance of 1 930 m and 1 940 m were selected as targets for an outdoor experiment. The results show that distance error was reduced by 58.3%; the feasibility of double thresholds processing method was proved. On the other hand, the results show the error probability on double thresholds has a minimum.