The modern nation-state is a very important and complicated political entity form in today's world. In the foreseeable future, it will still be one of the world's most general and dominant community pattern. Therefore, it has certain academic and practical significance to understand the basic situation of modern nation-state by analyzing and explaining its fundamental definition, core bearing consciousness and legitimacy construction. This paper suggests that modern nation-state as the most common pattern of modern political entity is the product of modern society, which ensuring the existence of the whole ethnic community and all individuals in it. The nation-state has to cohere and unite its people through the dual core of national identity and nationalist ideology. On one hand, national identity has become the operational foundation of modern nation-state and provided the most core cohesion for the establishment, consolidation and development for it. On the other hand, nationalism can be a kind of modern ethnic ideology and social political movement, and also a demand of economic system, a construction of social cultures and feelings. Only on this basis, the nation-state's autonomy and people's survival rights and unique values can be guaranteed, then the legitimacy construction of the nation-state can be realized. And the legitimacy construction of modern nation-state embodies in the enforcement of nationality policy, the protection of national sovereignty and the balance of international order. In the era of nation-state, we have to continuous concern both its broad connotation and its problem domain to grasp the essence of the modern society.