The key to realize the principle of legality is to interpret criminal law legitimately when settling criminal cases, and its way is not singular but plural. The substantive interpretation doctrine only sketches out a general picture though its objective, standpoint, limit and sequence. In order to attain the legitimacy of interpretation, procedural method should be considered as well as substantive method. That is, the prosecutor, the accused, the juror, and the judge should communicate and exchange respective legal opinion effectively and adequately. Besides, if the meaning of a word is ambiguous, the judge should give priority to formal rationality and interpret it in favor of the accused and alwsys keep the idea of protecting human rights in his mind.