Abstract: Applying Halliday's systematic functional linguistics as theoretical basis, this paper carries out researches on discourses in criminal lawsuits. Firstly, using actual courtroom discourses, the paper holistically investigates the modal expressions of lawsuit participants and their interpersonal meanings in a criminal lawsuit. It then analyzes the features of the modal expressions of the prosecutors and defendants, in an effort to unveil the general trend of lawsuit participants'use of modal expressions, illustrate that their choices of modal expressions are related to their respective roles and search for their legal explanations and proofs