Western scholars believe that criminal law should include not the defendant, but the country. Because, in a peaceful society, the most frequent and prone place of state power violence is criminal proceedings, which can legally deprive citizens of their property, freedom and even life. In the face of a powerful national machine, any individual is weak. Once the legitimate rights of the weak are not guaranteed, it will not only cause irreversible harm to the weak, but more importantly, it will lead to the abuse of public power and become a legal means to suppress citizens, then the levee of democracy and the rule of law will collapse. Therefore, the criminal law must first contain the state, and there are two main methods: one is to prevent the arbitrary introduction of the state punishment right through a legally prescribed punishment for a crime; The second is to establish the criminal defense system, to give individuals legal means to fight against state power, to protect the human rights of defendants from arbitrary infringement of the state power of punishment, and to promote the correct exercise of the state power of punishment. The legitimacy of criminal defense lies in that restricting power does not mean resisting power, and protecting human rights does not mean indulging in crime. Therefore, although the lawyer's defense behavior is directly reflected in safeguarding the interests of criminal suspects and defendants, the result is to safeguard the entire fair and just legal system. Bacon said in On Justice: "An unjust judgment is more serious than many illegal acts. Because these illegal acts only pollute the water flow, but unfair referees have destroyed the water source. " It can be seen that unjust national judicial acts are more harmful than citizens' criminal acts. Criminal defense is undoubtedly one of the systems to prevent water pollution. In China, the right to defense is a clear constitutional right, which is undoubtedly a footnote to the legitimacy of criminal defense.