Any academic research abides by a certain paradigm.The legal language study in China has experienced the "language-rhetoric-logic analysis, ""language style analysis, ", "writing type analysis"and "discourse analysis."This paper aims to propose a new paradigm: a paradigm of discourse analysis of legal language study.The legal language study based on "speech act"means that any "speech act"is a kind of human act, any "speech act"is part of a practice of an intentional action, and any rule of an"action"is determined by its immanent nature.Hence, analyzing the prescription of an act is a precondition to study a "speech act."Accordingly, we first give a sufficient definition of the action of "law", then draw a sufficient definition of "legal speech act"and logically differentiate constitutional rules from strategic rules of legal speech act.Based on these studies, we deduce the constitutional rules of legal speech act: legal code, formality, authority, consistency, attestation, time and readability.