The relationship of fuel pressure in fuel supply loop between the solenoid movement was analyzed at first. The results indicated that the consistency of the fuel pressure showed good. Secondly, the simulation model of fuel supply loop was established and validated by AMESim in which the relationship of the high pressure between electronic unit pump (EUP) and the inner of plunger chamber was obtained. Thirdly, the fuel pressure near to the EUP was analyzed, the time of fuel filling was calculated and the effect of it at different fuel supply pressures and speeds was discussed. Finally, the threshold speed of fuel filling process at different fuel supply pressures was analyzed, and the optimal fuel supply pressure was fixed which adapted to all engine conditions. Simultaneously, the effect of insufficient fuel filling was discussed. 