The theoretical inevitability of Xi Jinping thought on diplomacy is concentratedly reflected as the inevitable guidance for adhering to the objective truths of dialectical materialism and historical materialism, conforming to the complex and profound changes in the conditions of the world, the country, the party and the people in the new era, and adhering to the inevitable theoretical advancement of the innovation and development of Marxism in China. The theoretical ought to be of Xi Jinping thought on diplomacy should be embodied in the turmoil that has not been seen in the world in a century, and the prospects and applications for the development of major-country diplomacy with Chinese characteristics in the profound interaction between China and the world, and it provides a unique Chinese answer with wisdom and courage in the promotion of the noble cause of peace and development of all mankind. In Xi Jinping thought on diplomacy, the theoretical inevitability lays the foundation for the theoretical ought to be, and the theoretical ought to be deepen and enrich the theoretical inevitability. Xi Jinping thought on diplomacy is the organic unification of theoretical inevitability and theoretical ought to be.