In the absence of a specific international rule system to effectively regulate them at present, unilateral sanctions are increasingly being used frequently by certain countries. The vagueness of conceptual definition and the complexity of legal nature have led to difficulties in determining their illegality. In practice, the initiation of unilateral sanctions is often based on a country’s domestic law, which runs counter to the principles of international law and the basic norms of international relations. From the perspective of resolutions of the UN Security Council, certain sanctions obviously with unilateral nature may be defended as implementing resolutions of the UN Security Council on multilateral sanctions. From the perspective of the law of treaties, improper interpretation and application of relevant exception clauses in international treaties may provide excuses for the implementation of illegal unilateral sanctions. From the perspective of customary international law, unilateral sanctions beyond the scope of the countermeasures system, especially the legalization of countermeasures by countries other than the injured country, may lead to the abuse of unilateral sanctions. China should speed up the improvement of the counter-sanction legal system to effectively deal with the threats and damages caused by unilateral sanctions, and resolutely safeguard national sovereignty, security and development interests.