External gear pumps are mostly used in open hydraulic circuit, such as transmission, pressurization, fuel injection and lubrication, etc. Because of the low suction pressure of the pump, air will be mixed into oil inevitably, which has a negative impact on the hydraulic system. In some special occasions, such as high altitude hydraulic system, because the equipment works at high altitude environment, the suction pressure of the fuel pump (e.g. gear pump) will be lower than the ordinary atmospheric pressure, which will undoubtedly lead to underpressure suction of the gear pump and aggravate the cavitation phenomenon. In order to study the influence of high altitude working environment on the performance of gear pump, it is very important to fully understand the flow law of oil in gear pump and analyze the influence of oil suction pressure on its cavitation characteristics. In order to study the influence of suction pressure on cavitation characteristics of external gear pump, numerical simulation and visualization test method were used to analyze an involute external gear pump. The gear pump was widely used in fuel injection system. The simulation and test were carried out at 0.05MPa，0.10MPa and 0.15MPa, respectively. The vapor volume fraction distribution of the gear pumps internal flow field was numerically simulated. The actual flow state, bubble size, number of bubbles and cavitation degree in the gear pump were recorded and observed. The results showed that at different suction pressures, the fluid in the gear pump would have different degrees of cavitation. The cavitation intensity from strong to weak was as following: vortex flow, veil flow and bubbles. With the increase of suction pressure, the number and volume of bubbles of the oil would be decreased gradually, which would reduce the max value of vapor volume fraction and cavitation degree of the oil, made the flow state of the oil more and more stable, and then improved the continuity and stability of gear pumps outlet flow.