Space debris distribute in a large range in the space, and their size are very different. For the purpose of detecting the small space debris, the space-based laser radar is used. The characteristics of the background radiation in space are analyzed at first. Then, according to the analog and digital detection theory of laser radar, with avalanched photodiode as the detector, the minimal detectable laser powers are calculated. Based on the radar equation, the detection ability of laser radar is got. The result indicates that the space-based laser radar is effective to detect the small space debris in a specific range.