Cavity pressure is an important issue in injection molding technology, as it determinates the quality of final parts. Compared to traditional injection molding, Cavity pressure during the filling and post-filling stages is pulsatile in dynamical injection molding. Frequency and amplitude of vibration are important factors to cavity pressure. Low-Density Polyethylene (LDPE) disc-samples were injection molded by way of traditional injection molding and dynamic injection molding. Meanwhile, by using the self-made pressure sensor, cavity pressure change with real-time is acquirered. The test results show that vibration frequency of cavity pressure is equal to vibration frequency of piston rod. In addition, it is show that vibration amplitude of cavity pressure, average cavity pressure and the maximum cavity pressure increase with increasing vibration amplitude and/or frequency of ram.