The invention relates to a turntable clamp used for the welding industry, in particular to a pre-deformation hydraulic clamp for a turntable welding system. A mounting base of the hydraulic clamp is in a shape of an octagonal prismatic table, on which pressing components, a hydraulic station, a workpiece supporting component, and a positioning bolt are arranged. The pressing components are symmetrically distributed radially, a main support of which has a frame structure and fixed on a bottom panel of the pressing components. The bottom panel is arranged on a guide rail which is fixed with the mounting base. A mould lifter is arranged on the guide rail. An oil cylinder is fixed in the main support, and is connected with the hydraulic station by an oil line. The top of the main support is sleeved with a pressing arm through a fixing shaft, and two ends of the bottom surface of the pressing arm are provided with a lowest protrusion respectively. The protrusion at one end is contacted with the top of the oil cylinder, and the protrusion at the other end corresponds to a pressing plate therebelow. The pressing plate is arranged on a pressing plate support which is fixed with the mounting base, and an adjustable pad is assembled between the pressing plate and the pressing plate support. The pre-deformation high-efficiency hydraulic clamp can position precisely and clamp efficiently, has low deformation of workpieces after welding, and improves the product quality and the production efficiency.