From the perspective of traditional political science, this article presents a brief introduction of the structural characteristics of the EU/ EC institutions, and shows the key factors of the EU mixed constitution of federalism and confederalism through the comparison of American and German federalism models. This article analyses the reforms of EU constitution, especially the introduction of qualified majority rules on more decisions, and points some difficult problems that are ignored usually such as deliberative decision rules and centralization. It seems that rather than the mixture of federalism and confederalism in general institutions, the modern federal structure of dual sovereignty and dual citizenship would be chosen to resolve these problems.