Abstract: The current situation of the space debris environment and its hazards to spacecraft are introduced in detail. The necessity of an active debris removal for maintaining the stability of the space debris environment is discussed. The keynote of space debris changed from protection to mitigation, then to active removal, and to the end of clean space. The techniques of the active debris removal are analyzed. These techniques are still developing, among which the active debris removal using the space-based laser seems to be more promising.