With the acceleration of the democratization process in countries, the study of Constitutionalism has become deeper and deeper. However, on the research, two questions must be clearly. To begin with, why should we need Constitutionalism? In addition, what kind of special way should be used in the study process. As is known to all, Britain is the root of Constitutionalism. There is no constitution code, instead, the Constitutionalism has come true. Therefore, when finding many countries create and modify their constitution codes, whereas they can’t make the Constitutionalism come true. Thus, a question what guarantees the Constitutionalism may spring up. With the problem, the author starts the problem from the Constitutionalism of Britain to study the establishment of the process of Constitutionalism tradition and its psychoanalysis, in the hope of which can contribute to the development of Chinese Constitutionalism.The article is divided into four chapters in time’s order and through the historical narration of the medieval Britain.Chapter I introduces the traditional initiation of British Constitutionalism, the first part, taking the Crown Polity of the Anglo -Saxon as study object in order to reveal the origin of Monarchy. The second part, introduces the Witenagemot about its formation and role and exhibits its historical status in British political system, paving the way for the British political consultative tradition. The third part of this chapter introduces the formation and role of Customary Law to show its significant effects in the restriction of Crown, forming the foundation of tradition-the King controlled by law.Chapter II describes the growth of the tradition of British Constitutionalism, in the first part, the author takes the history of the King polity of Norman Dynasty as an object to state how to strengthen the Monarchy under Feudalism. Among the measures, not only inherited the characters of expansion of Monarchy in Anglo -- Saxon period, but also established a unique Enfeoffment. Comparing with the system of absolute Monarchy in other European countries at that period, it was outstanding. The second part of the chapter gives an introduction of the formation and role of Magnum Concilium, which shows its historical meaning in British political system. It inherited the great quality of the Witenagemot. What’s more, it had developed its own unique characteristics, practiced and developed the traditions of British polity. The third part exhibits the special charm of Feudal Law through the introduction of Feudal Law’s formation and role, namely its compromise. The author focuses on the analysis of the restrictions of Crown in the Magna Carta, which should be useful to the recognition of Feudal Law.Chapter III describes the establishment of Constitutionalism tradition in Britain. In the first part, The author takes the political system of Angevin Dynasty as an object to study the contribution of Henry II in strengthening the Crown, one should be noted is at the period of Henry II, the Crown was not be strengthened arbitrarily, it started to pay attention to the respect of people’s customs, pursuing the policy of dealing with affairs peacefully. The second part of it reveals the milestone meaning of Congress in British political system through the introduction of its formation and role, ensuring the British political consultative tradition by system. In the third part, the author introduces the formation and role of the Common Law and states the accord between the Common Law and the British Constitutionalism.Chapter IV analyzes the spirit of British Constitutionalism Tradition. With the historical description of above, the author refers it is "Political Consultative Tradition" and "Tradition of the King controlled by law" that makes the British Constitutionalism Tradition establish ,which can be seen in The essence of the British Constitutionalism written by Dicey. However, there is a foundation about such political and legal cultures called The balance of Constitutionalism. Under the guide of the spirit of Freedom and Democracy, The balance of Constitutionalism has become a historical trend.In a word, the author takes the Crown Polity as the background, analyzing the evolution of British political consultative tradition and the development of the rule of law, studying the relationship between polity and law.