Electronic unit pump system is a type fuel injection system with high injection pressure which can be applied to control fuel quantity and timing of insection flexibly. The solenoid valve in the electronic unit pump (EUP) controls pressure buildup within the highpressure fuel line. When it is open, highpressure fuel transfers with high velocity from the narrow crosssection into the lowpressure fuel line. During its operation, the flow in the field of solenoid valve is quite complex that affects the performance of EUP. In order to study the change characteristics of transient flow fields in poppet valve downstream area during the oil return process of an electronic unit pump injection system, a diaphanous glass window was designed in the pump. Highspeed photography was introduced to visually investigate the transient flow fields. By means of experiment data, extracted the cavitation image boundaries and calculated the brightness of the cavitation bubble clouds, the cavitation periodic time and the change of brightness were analyzed and calculated. The test results show that with the increase of the cam rotational speed, the average cavitation periodic time was decreased almost linearly, and the average brightness was increased gradually. At the idle condition (500r/min), the average cavitation periodic time and the standard deviation of fluctuation are maximum, which are 4.5ms and 0.15 respectively. Furthermore, the simulation was conducted based on the test data such as the pressure and the displacement of valve core. The simulation results of cavitation phenomenon occur along opening process of valve. In the process of reciprocating motion of valve core, several swirling regions appear in the flow field of poppet valve downstream area, and keep constantly changing. The streamlines have significant influence on swirling regions.