International institutions are the most significant research topics in the field of international politics during the process of globalization and integration. International institutions refer to the agreed principles, norms, regulations and decision-making process of some international actors in a certain domain, which should be based on strong legitimacy and identification in order to be applied in specific internal affairs; otherwise, it would be invalid. Therefore, the legitimacy of international institutions has become a vital research topic in international relations. Although Political Psychology is, to some degree, a new subject, it has a profound impact on the analysis of legitimacy, through which the legitimacy of European Union could be explained.