The invention relates to a synchronous rectifying device for a hoist, belonging to the technical field of the hoist. Two sides of a sealed mercury box are symmetrically communicated with two curved tube; the upper end of each curved tube is communicated with air; and each curved tube is internally provided with a mercury level sensor. The mercury box is filled with a right amount of mercury, the center of the mercury box is fixedly connected with the center of a valve with two suspension points, the valve is provided with the hydraulic hoist, and the signal output ends of all the mercury level sensors are connected with oil control work circuits of two hydraulic cylinders. The two hydraulic cylinders are controlled to work synchronously through a mercury balancer and the mercury level sensors, thus the opening and the closing of the large valve is smooth and steady, the synchronization accuracy of the two hydraulic cylinders is high, and the manufacturing cost of the hydraulic hoist is low.