The utility model relates to a clamping device of a welding structural piece used for the machinery manufacture of a coal mine, in particular to a manual hydraulic clamping device used in a manufacture welding working procedure of middle grooves of coal mine scraper conveyer parts. At present, in a welding production working procedure of the middle grooves of the coal mine scraper conveyer parts, a spiral clamping mode is still adopted for positioning and clamping middle grooves of work pieces on a welding positioner to finish welding in different positions. Because the work pieces are incontinent to mount and clamp at low speed, when the work pieces of different models are welded, the prior clamping device is unfavorable to adjustment, and the operation difficulty is increased. The clamping device utilizes a manual hydraulic jack and a lever theory, stress arms are mounted on a support fulcrum of the lever by a pin shaft, both ends of a jack oil cylinder are respectively hinged on the stress arms and the inner side support, and tension springs are mounted at both ends. When the utility model is in use, oil is poured in the oil cylinder by manually lifting a rod so that piston rods of the oil cylinder extend out to drive the stress arms to rotate and clamp a work piece through a movable mandril. The device has time-saving and labor-saving operation, safety and reliability.