The invention discloses an optical-fiber-guiding weak-laser coaxial composite welding gun, and relates to the field of laser-TIG composite welding guns. The optical-fiber-guiding weak-laser coaxial composite welding gun aims at solving the problems that in traditional single-welding-wire and single-heat-source welding, weld joints are wide, welding deformation is big, the welding speed is low, and the fusion depth is small. An electric arc is directly generated through induction of laser, a tungsten electrode is manufactured by being divided into multiple segments, an optical fiber is arranged in the tungsten electrode, and the laser is introduced into the tungsten electrode through the optical fiber and focused through a collecting lens; the length of the tungsten electrode and the length of the optical fiber are regulated through a tungsten electrode length regulator and an optical fiber regulator, the exterior of the optical fiber is provided with a silicon dioxide heat insulation coating, and a spiral cooling water channel is lathed on an inner sleeve of the composite welding gun. According to the optical-fiber-guiding weak-laser coaxial composite welding gun, the theory that low-energy laser assists electric arc welding is utilized, the laser-TIG coaxial composite design is adopted, the structure is simple and practical, not only can the basic requirements of a common welding gun be met, but also the high-precision welding electric arc very easy to ignite can be obtained, the welding production efficiency is improved, and the production and maintenance cost is lower.