The invention relates to a hydraulic catch cam, which belongs to a mechanical hand. The mechanical hand comprises solenoid valves (1), (2), one-way back valves (3), (4), a piston (5), an inner cylinder body (6), an outer cylinder body (7), a hydraulic oil tank (8), a seal ring (9), an upper connecting link (10), a lower connecting link (11), a gripping claw (12) and a connecting axle pin (13). The invention has the advantages that the structure is simple, the control is automatic, the stretching is convenient and flexible, and the grabbing action, the releasing action and the reset action are finished by utilizing dead weight. The invention can be manufactured into specifications and models with different sizes to be suitable for different purposes, the invention not only can be manufactured the hand of a mobile robot, but also can be manufactured the fittings of a field working machinery which is used for substances in traveling, craning, fishing, deep pit grabbing, deep well, deep sea and a floating substance and a blocking substance in front of a fishing and grabbing cement factory and a hydropower station trash rack.