In order to seek a simple follower part movement law of the cam mechanism, based on the discipline of defining inverse hour as the positive direction, the cosine, sine acceleration and 5-order polynomial law are analyzed and the characteristic of rigidity and flexibility attack are studied. The forming of movement law with 4-order polynomial is proposed. Cutting the raise course into two parts, the five undetermined coefficients can be defined by 5 boundary condition of the first part. The velocity drawing of the two parts is mirror symmetry around the cut point. The acceleration drawing of the two parts are center symmetry around the cut point. There is neither rigidity nor flexibility attack under the new movement law. Its velocity and acceleration drawing are smooth and its sensitive coefficient on the cam acceleration is more perfect. So the new movement law has a perfect dynamic property. Contrasting to the other law with the same characteristic, the new law is more simple and practical.