The invention relates to the technical field of plastic bonding tools, in particular to a handheld welding device and a reinforcing piece thereof. The reinforcing piece is in a rod shape, is made of ahard electric heating material, and comprises two deformation parts and a connecting part which is connected with the two deformation parts, wherein the longitudinal section of each deformation partis rhombic, each deformation part comprises two points and two blunt ends, and the two ends of the connecting part are correspondingly connected with one points of the deformation parts; the handheldwelding device comprises a shell body and two clamping units which are located in the shell body and are oppositely arranged, wherein the shell body is in a U-shaped with a concave part, and the clamping units comprise sliding blocks connected to the surface of the shell body and pressing plates connected with the sliding blocks; and the pressing plates are positioned in the shell body, the two ends of each pressing plate are correspondingly connected with a welding tube and a limiting block, and a gap is formed between the two limiting blocks. The handheld welding device and the reinforcing piece thereof aim to solve the problems that in the prior art, plastic plates are prone to weld penetration, the bonding surface of the plastic plates is not flat, and welding needles are required to be removed in the follow-up process.