The electronic unit pump system, which is widely applied to the heavy-duty diesel engine, belongs to the pulsating high pressure fuel injection system, and the fuel pressure fluctuations have an essential influence on the spray and combustion in the internal combustion engine. Pressure fluctuations are always aroused by the motion of actuators, such as the injector, so it is also an advantage for fault diagnosis and feedback control to ascertain the relationship between the pressure fluctuation and the motion of the actuator. In this study, experiments and 1D simulation were carried on to investigate the fuel pressure fluctuation characteristics and their connection to the transient motion of the needle valve in the injector. Four fuel pressure characteristic points corresponding to the start of opening time, the x% opened time, the x% closed time and the fully closed time of the needle valve, are determined on the fuel pressure curve, where x relates to the flow area near the needle valve and the total area of orifices. The conclusions are that 1) the point of starting to decrease on the fuel pressure rise rate curve is defined as the characteristic point of the start of opening time, and 2) the trough of the pressure fluctuation at the rising stage of the fuel pressure curve is defined as the characteristic point of the x% opened time, and 3) the trough of the pressure fluctuation at the declining stage of the fuel pressure curve is defined as the characteristic point of the x% closed time, and 4) the peak of this pressure fluctuation is defined as the characteristic point of the fully closed time. Moreover, a simple case is given to validate this research.