In order to improve shearing quality, the structure of rolling-cut shear (RCS) should be designed carefully to guarantee a pure rolling condition of the top blade and the amount of scissors overlapping (ASO). This paper describes an investigation into the work mechanism of RCS. A parametric model of RCS has been established by using ADAMS software. The influences of the ASO and the horizontal displacement of the top blade (HDTB) are also discussed. The indications are that the length of crank, guide rod and link, initial phase angle, the position of gyration center for guide rod and the radius of the top blade arc are key factors to the ASO and HDTB. Moreover, those structure parameters that influence the ASO and HDTB are selected as design variables. An optimal mathematic model has been established by using two objective functions that are ASO and HDTB in the cutting process. This optimization model has been used successfully in the development of RCS 3800 to obtain reasonable mechanism dimensions for improving shearing quality. The study results provide a new idea for designing the RCS.