The flow characteristics of solenoid valve region of electronic unit pump (EUP). EUP was rebuilt by using optical visualization observation. A diaphanous glass window was designed in the pump, and the transient flow in the solenoid valve outlet was observed by a high-speed camera. By gray processing of pictures, the brightness curve within the control valve region was acquired. Results showed that cavitation occurred in control valve of EUP and formed the two phase flow phenomenon. The formation of cavitation had two factors, one was due to flow region in control valve where high-speed fuel flow within was quiet narrow, another reason was that the control valve hit the baffle which caused vibration and impingement, hence the fuel was extruded.