Provided is a method for welding a high-voltage power transmission line grounding strap with thermit. A mould is placed on a flat ground. The mould has a rectangular mould body, and the upper surface of the mould body is provided with a step-shaped through hole. Two grounding straps needed to be welded are lap-connected on two sides of a wide groove of the through groove. A gap between the two grounding straps is filled with the thermit. After flattening and compacting, an igniting agent is piled on the thermit and ignited. After the thermit in the mould is fused, the two grounding straps are welded. The operation method is simple without an external power supply or a heat source so that welding time can be greatly shortened and personnel labor intensity is greatly reduced. The welded grounding straps are high in intensity of welding points and are not affected by corrosive products so that welding requirements of the grounding straps can be met by electromechanical property.