A mechanism for detecting the lifting capacity of a hydraulic lifting device of a tractor comprises a hydraulic cylinder, a slide rail, a pulley, a fixing device at the upper end of the hydraulic cylinder, a cylindrical pin with holes, a fan-shaped safety pin, a Y-shaped hooking rod, trapezoid slots and slot holes. The Y-shaped hooking rod and the hydraulic cylinder are connected into a whole mechanism by the cylindrical pin with holes and the fan-shaped safety pin. The hydraulic cylinder is equipped with the pulley, and can move freely on the slide rail through the pulley. The telescopic endof the hydraulic cylinder is connected with the Y-shaped hooking rod through the fixing device, the cylindrical pin with holes and the fan-shaped safety pin. The hydraulic cylinder is loaded with a weight at the bottom thereof. The hydraulic cylinder is perpendicular to the slide rail when the hydraulic cylinder is still. The two ends of the upper part of the Y-shaped hooking rod are provided withthe trapezoidal slots, and the slot holes are arranged in the walls of the trapezoidal slots. Semi-automatic fast aligned hooking can be realized. The working intensity of workers is reduced. The safety factor of hooking operation and the efficiency of tractor pre-delivery inspection are improved.