In this study, the nonlinear pressure-flow characteristics of a spring-loaded pressure relief valve (PRV), which is used in the automotive fuel supply system for pressure control is analyzed, and its characteristics are improved by means of geometrical modifications of the valve structure. Given the complexity of the coupling mechanism between the valve internal flow characteristics and spring system, a quasi-steady computational fluid dynamics (CFD) method is introduced to predict the nonlinear pressure-flow characteristic curve of the valve and the accuracy is validated by experimental data. The total hydraulic force on the valve spool and diaphragm are divided into three parts according to the loading position and the correlation between the internal flow characteristics, hydraulic force, and pressure-flow characteristics of the valve are explained by CFD analysis and visualization. The results show that the quasi-steady CFD method can accurately predict the trends of the valve nonlinear pressure-flow characteristic curve, which is mainly determined by the hydraulic force produced in the middle chamber of the valve. When the valve opening reaches a certain value, a main vortex would be formed in the middle chamber and lead to the sudden increase of hydraulic force which causes the fluctuation of the pressure-flow characteristic curve of the valve. It is also found that the toggle point for the flow regimes seen in the valve is affected by the geometric structure of the middle chamber and the pressure-flow characteristics can be improved by the round corner size modification.