In order to investigate the influence of the back pressure on the solid discharge rate of the U-valve, a setup with adjustable back pressure was established. Experimental results revealed that the solid discharge rate, as well as the effective aeration rate on the solid discharge rate decreased with increasing the back pressure. Both the back pressure and the orifice height had significant influence on the discharge behavior of the U-valve. When the back pressure fluctuation was significant, the operation of the U-valve was more stable with a smaller orifice height. The orifice height had a suitable range to obtain a stable discharge rate, i.e., the orifice height was less than 0.11 m, the back pressure was less than 7 kPa, the back pressure was greater than 13 kPa, and the orifice height was more than 0.15 m. In addition, the solid discharge rate was correlated with the back pressure, the aeration rate, and the orifice height. The average relative error was 20.9% between the calculated and the experimental data. Based on this correlation, the reasonable range of orifice height was calculated to provide the guidance for the design of U-valves.