The invention relates to the technical field of train body strength tests and discloses a test loading device of a fuel motor train unit under-train fuel tank. The test loading device comprises a loading plate, a loading oil cylinder, a force sensor, a simulation fuel tank frame and a counterforce frame, the simulation fuel tank frame is fixedly mounted on a fuel tank mounting seat of a train body, the counterforce frame comprises a frame body and a counterforce plate, the frame body is fixed on a testbed, the counterforce plate is arranged on the frame body, the loading plate is arranged below the counterforce plate and connected with the simulation fuel tank frame through a connecting component, the loading oil cylinder is placed between the loading plate and the counterforce plate, and the force sensor is arranged between the loading oil cylinder and the loading plate. The test loading device is simple in structure, convenient to mount and demount, quick and accurate in loading, capable of repeatedly loading a suspension area of the under-train fuel tank in short time and guaranteeing a loading gravity position to be at the center of a loading area, high in loading accuracy and good in repeatability. A hydraulic loading mode is adopted, so that the test loading device is uniform in loading, capable of quickly adjusting and controlling loading range and free of sudden-change load.