As the temporal dimension for examining the legal relationship in international law, the critical date is the date when the legal relationship of a dispute crystallizes. The critical date plays an important role in recognizing the facts of a dispute and has significant impact on the admissibility of evidence. Some Japanese researchers regards the date when Japan renounced Nansha Islands after the World War II as the critical date of the territorial sovereignty dispute of Nansha Island , and accordingly conclude that the territorial sovereignty of Nansha Islands is disputable. This paper holds that the critical date is only applicable to the legal relationship between the disputing parties, and therefore has nothing to do with the third party. The illegal acts of Japan can neither serve as a basis for determining the critical date of Nansha Islands, nor change the legal status of Nansha Islands as Chinese territory.