The soundness and perfection of criminal defense system is an important symbol of a country's democratic and scientific criminal procedure. Criminal defense refers to the refutation and defense of criminal suspects, defendants and their defenders in order to prove their innocence and light crimes and protect their legitimate rights and interests. Its essence is to give the defendant an opportunity to speak for himself and make him participate in criminal proceedings "meaningfully" and "effectively" as the subject. Through criminal defense, exercising the right of defense plays a beneficial role in the formation of the judge's final judgment. The right of defense is the litigation right that criminal suspects, defendants and their defenders enjoy according to law to defend the investigation and accusation of criminal suspects and defendants. It is aimed at offensive charges and is the most basic and core litigation right of the defendant. It is the foundation of criminal defense system. Without recognizing the right of defense of criminal suspects and defendants, there can be no criminal defense system. Criminal defense system is the general name of a series of rules about the right of defense, defense types, defense methods, the scope of defenders, the responsibilities of defenders, the rights and obligations of defenders and so on. The defense system is the guarantee of the right to defense, and various defense systems are set up to ensure that criminal suspects and defendants fully and correctly exercise their right to defense.