Rapid pressure unloading is essential for the back pressure assembly of injector test systems. In this study, a high-speed unloading valve was developed for the back pressure assembly of an injector test system, and its pressure unloading characteristics were investigated through coupled numerical simulation and experimental testing. An electromagnetic–mechanical–fluid coupled dynamic model was established to describe the interactions among electromagnetic actuation, valve core motion, and hydraulic forces. Three-dimensional CFD simulations were performed to investigate the steady flow and hydraulic-force characteristics, followed by a two-dimensional transient model for analyzing the hydraulic force during valve closure. A coupled Simulink–AMESim model was then established to evaluate the system-level pressure unloading response and compared with experimental measurements under several initial pressure conditions. The CFD results reproduced the variation in flow rate with pressure differential, while sensitivity and uncertainty analyses showed that the quantified factors could not fully account for the discrepancy in absolute flow rate. Three-dimensional CFD revealed both axial and radial hydraulic forces, while 2D–3D comparison supported the use of the two-dimensional model for transient axial-force analysis. The system-level simulation and experimental results exhibited comparable pressure unloading characteristics within the investigated operating range.