Abstract: Kant's philosophy, including his theory of knowledge, moral philosophy and aesthetics, forms a complete system. By contrast, His political thought is fragmented, which means that the study of Kant's constitutional thought should be in the context of his philosophy. Kant's constitutional thought has his own logic. It can be said that his constitutionalsm is based on the general ethics and morals. This is the thought that is reflected in Kant's moral philosophy. Kant used the "freedom" idea as a clue in moral philosophy to argue his constitutional thought, including the possibility, necessity, principles and realization of constitutionalism.