The invention relates to the technical field of positioning jigs, in particular to a positioning jig for a welding gun, the positioning jig comprises a bottom plate, a transverse sliding rail, a longitudinal sliding rail, a sliding plate, a limiting mechanism and adjusting assemblies, the limiting mechanism comprises a plurality of limiting assemblies, and each limiting assembly comprises a support, a first clamping piece, a second clamping piece, an adjusting seat and a lifting screw. During welding, the sliding plate can move along the transverse sliding rail, the limiting assemblies can move along the longitudinal sliding rail, a user can adjust the front-back position and the left-right position of the multiple limiting assemblies according to requirements, and after a welded object is inserted into the clamping cavity, the first clamping piece is moved downwards, and the welded object is clamped and then welded; and a welded object can be effectively prevented from moving during welding, and the welding accuracy is improved.