By experimental studies for the single EUP on test bench, the low fuel supply pressures near the EUP and the high pressures in high pressure fuel pipe at different cam speeds were acquired. The high and low pressure curves of the EUP’s cyclic variation circles 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 backward flow of high pressure fuel appears in low fuel supply loop. The above two factors cause the negative pressure at low fuel supply loop, and leads to the cyclic variation. Increase in fuel supply pressure and the fuel supply pipe line’s flow area is advantageous in reduction the cyclic variation of EUP. 