The utility model provides an oil jack and a loading system. The oil jack comprises an oil cylinder and a piston. The oil cylinder comprises an oil cylinder bottom, an oil cylinder wall and an oil cylinder cover. The piston comprises a first part and a second part, and an oil return cavity is enclosed by the oil cylinder cover, the first part, the oil cylinder wall and the second part. An oil inlet cavity is enclosed by the oil cylinder wall, the piston and the oil cylinder bottom. Oil in an oil tank flows into the oil inlet cavity through an oil inlet pipeline, and upward pressure formed at the bottom of the piston pushes the piston to move vertically and upwardly. The oil in the oil tank flows into the oil return cavity through an oil return pipeline, downward pressure formed on the upper bottom surface of the second part pushes the piston to move vertically and downwardly. The oil jack solves the problems that existing oil jacks fail to meet the requirement for overload of destruction tests and several oil jacks fail to ensure uniform stress on the surface of a model when used as the same time.