The liquid ramjet system uses the overload protection valve to prevent fuel backflow. The structural dimensions of the valve can affect hydraulic resistance and reverse closure flow greatly. To obtain the most appropriate structural parameters of the overload protection valve, the numerical simulation was conducted to analyze the variation situation of flow patterns and hydraulic resistance. The research results show that the baffle angle and the outlet angle significantly impact on the hydraulic resistance and minimum closure flow. Nevertheless, the length of the straight cylinder in the valve case has few effects on the hydraulic resistance, but its size directly affects the value of the outlet angle. The critical closure flow and the fitting formula of flow characteristics were obtained through numerical simulation and validation experiments. The most appropriate parameters for the overload protection valve are confirmed, which can be used to guide the engineering design of this kind of valves.