After the end of the cold war, separatist movements emerged again, and many countries faced the crisis of ethnic disintegration. In the design of the political system, the United Kingdom. and Spain are classified as unitary states. However, there are three different degrees of autonomy in administrative regions within the UK. Spain is divided into 17 autonomous regions, but not every autonomous region has the same power, thus changing the unitary state power distribution between central and local governments. The power relationship between central and local is not a zero-sum game. In the case of unequal powers, how to respect each other's position, resolve disputes through negotiation and negotiation, and achieve a certain degree of balance of power from the realistic angle of the distribution of power and benefit, is a question that the modernized country must ponder. Through the case studies of Scottish and Catalonia independent referendum, this paper explores the principle of "decentralization" and "federalism arrangement" in unitary states, and analyzes and explains the process of institutional change under the two opposing systems and concepts of centralization and decentralization.