Minimum detectable signal-to-noise ratio was an important parameter which had influence on tracking radar range target, as signal-to-noise ratio reached threshold, detector established identification and radar tracking started. Aimming at the problem of tracking radar range decline, the relation between minimum detectable signal-to-noise ratio and radar range was analized, combined with fault tree analysis, the root cause and solution was presented. The proven flight test result show that problem analysis was corret and solution was effective.