This paper analyzes the reasons for the heavy milling vibration generated from the processing water chamber head under a large overhanging, and compares the machined surface of workpieces based on field processing experiments. An eccentricity runout model of the heavy milling cutter, a slight deformation model of the cutting tool and spindle system, and a rigidity model of the feeding system were established, and the effect of the instantaneous cutting thickness was analyzed. Meanwhile, the mathematical model of the heavy cutting force and milling trajectory model were established to study the heavy milling system vibration and rules. The heavy milling prediction results agree with the experimental results. Thus, this research will lay a foundation for further study on the vibration characteristics of heavy milling