Abstract: In ancient China there was a serious crime called as Ought Not To Do, which came into its full being in Tang Dynasty. The paper discusses a case on Ought Not To Say crime in a great work of the ancient China legal system cases about Dragon and Phoenix, analyses how the Ought Not To Do crime came into being, criticizes the judge-centered legal system in the feudal society, and verifies the role the legal principle of crimes played in the legal system development history in China.