Abstract: This article studies the rail pressure under the influence of key component in high pressure common rail diesel engine and its control in extreme conditions. The effect of flow quantity, fuel leakage and characteristic of solenoid valve to rail pressure in high pressure commonrail system was analyzed. The rail pressure could be controlled by adjusting the flow of delivery valve, through a precontrol method. Experiments show that this approach is very flexible, and it makes the pressure control fast, stable and exact, especially in complex situation with the fierce change of injection quantity.