Since the end of the Cold War, new international security circumstances and challenges have emerged. The most threat to international peace and security is humanitarian catastrophe within states as a result of ethnic conflict, civil war or state failure. To face these challenges, the old thinking about maintains of international peace and security must change. If we continue to observe non-interference in internal affairs and prohibition of use of force, how to deal with those humanitarian disasters has happened in Bosnia, Somalia, Rwanda, Kosovo, East Timor, Congo, Sudan and Haiti? For stopping conflict to dissolve or alleviate such humanitarian crisis, the claim of the New Interventionism is to interfere in internal affairs of those desperate states by use of force. As the United Nations Secretary-General reports and the Security Council resolutions, the New Interventionism not only based on humanitarian reason, not only also stopping conflict, but also to integrate preventive conflicts and post-conflict reconstruction, becoming the maintenance of international peace and the peace complete strategy. In the principle of legality and legitimacy, the New Interventionism is to interfere in internal affairs by force, for preventing and resolving conflict, and post-conflict rebuilding. It breaks the principle of non-intervention in a sovereign state, and expands the scope of use of force; at the same time, it tolerates use of force based on legitimacy. This paper will discuss the transformation of non-intervention at first, and then elaborating the central issue of this article: the legality and legitimacy of the use of force.