The parameters of the high pressure oil pipe have a great influence on the he high pressure injection characteristics of EUP and there are a lot of researches about this. But the researches about the fuel supply loop are less, in fact, the fuel supply loop is equally important. The coupled experiment of high-low pressure fuel pipe of the diesel's EUP was conducted. The low fuel supply pressures near the inlet of EUP, high pressures in high pressure fuel pipe and the instantaneous injection quantity at different cam speeds were acquired by experimental studies for the EUP on test bench. The influences of return fuel from high pressure fuel pipe on the fuel pressure fluctuation of fuel supply loop, fuel supply loop on characteristic of high pressure injection were analyzed. Test results showed that when the cam shaft speed is very high the supply pressure instantaneous decreased as the plunger move downward, and the pressure wave which results from the back ward flow of high pressure fuel appears in low fuel supply loop, and the pressure is lower near the inlet due to the flow resistance in the pipeline. These above three factors cause the negative pressure at low fuel supply loop, even there is cavitation at low fuel supply loop. This has a big adverse impact on high pressure injection characteristic of EUP. E.g. it leads to the cyclic variation, increases the damage to the parts and shorten the service life of the EUP when the bubble bursts in the fuel. Increase in fuel supply pressure and the fuel supply pipeline’s flow area is advantageous in improving the performance of EUP.