Abstract: A hydraulic wrench driving plate of unknown material fractured at the contact part with the piston rod. By the physical and chemical analysis, the fracture cause of the driving plate was analyzed and the improvement measures were proposed. The results show that the material of the driving plate was X2NiCoMo18-8-5 maraging steel. A fatigue fracture of the driving plate occurred. The scratches were produced at the contact part of the driving plate with the piston rod, resulting in the stress concentration, and the crack source was further formed at the scratches under the force of piston rod. And then the cracks propagated continuously, finally leading to the fracture. It was suggested to increase the edge radian of the contact part between the hydraulic wrench driving plate and the piston rod, make the surface of the contact part smooth and assemble the piston pod correctly to avoid the scratches.