Abstract: Kant's redefinition of concept, not only is to resolve the problem of different perceptions about the source of concept pit rationalists against empiricists, but also is a necessary requirement of "the epistemological reversal". The task of this redefinition is that concept is redefined as a pure form of reason's ability (in the broad sense). Because there are different levels of the reason's ability and every different level has its own object and scope, its concept has different functions and transcendental contents correspondingly. A pure form of understanding is the category of understanding and a pure form of reason is the transcendental idea. The category of understanding as a pure form has provided conditions of universality and inevitability for empirical cognition. The transcendental idea as a pure form is not merely a subjective principle of a systematic structure of empirical cognition, but means that a person is a free existence and has provided a rational basis for a person as a free existence.