Previous Investigations show that it is not easy for the freestream to enter a submerged inlet with flush-mounted planar side entrance. To improve the performance of this type of inlet, three important entrance parameters, side edge angle, ramp angle, and characteristic parameter of aft lip, are introduced in this paper, and their effects on the performance of the inlet are studied by both experiments and computational fluid dynamics. Based on the results, a submerged inlet with a trapezoid entrance is designed and tested over a wide range of flow conditions. Results indicate that the designed submerged inlet can give excellent performance at the design condition and provide satisfactory performance over a wide range of test conditions. By using a numerical analysis, the influences of the freestream Mach number, the angle of attack, yaw, and Mach number at the exit on the performance and flow structure of the submerged inlet have been obtained.