The invention discloses an inclinable single gearbox electric power closed-loop loading test system and aims at solving the problem that reliability of D-series high-speed train gearboxes cannot be tested during actual running. The system comprises a master supporting assembly and a closed-loop torque loading test assembly mounted in the middle of a simulation test bed, the master supporting assembly comprises two middle supporting seat assemblies and four master supporting assembly bodies which are symmetrically distributed on two sides of the closed-loop torque loading test assembly, an assembling shaft and a connecting plate with an elongated bolt hole are processed symmetrically on two sides of a welding platform in the closed-loop torque loading test assembly respectively and used for being in matched connection with a middle supporting seat and a master supporting upright, and a hydraulic cylinder assembly is mounted below the welding platform. Extension and contraction length of a piston rod is adjusted by controlling pressure of oil injected into a hydraulic cylinder barrel so as to adjust inclination angle of the closed-loop torque loading test assembly.