To obtain high-quality plastic products, an industrial volume-pulsatile injection-molding (VPIM) machine is developed to achieve high screw displacement amplitude. However, it is difficult for engineers to optimize vibration parameters and improve equipment due to lack of a comprehensive understanding of the coupling correlation between amplitude and frequency, which seriously hinders the development of VPIM technology and machine. To address the challenge, this paper investigates the amplitude–frequency characteristics during the volume-pulsatile packing process. According to the working principle of this machine, the dynamics model of injection screw is established and equivalent to a mass-spring-damper system. Based on the dynamics equivalent model, we deduce the forced vibration response of screw displacement, and the vibration responses of screw displacement and oil pressure under the hydraulic system. An oil pressure control model is proposed and numerically solved. Experimental and theoretical results reveal the effects of material properties, machine components, and control system on the amplitude–frequency characteristics during the volume-pulsatile packing process. For injection-molding grade materials, the reductions in oil pressure amplitude and screw displacement amplitude are attributed to the inertial delay of the hydraulic system in this paper, rather than the viscous resistance of materials in the previous perception. The response speed of the servo control system to pressure should be improved to enhance oil pressure amplitude and screw displacement amplitude. This paper enriches the basic theories of polymer vibration processing and points out the direction of equipment improvement.