The invention discloses a multi-purpose single-contact-tube twin-wire MIG/MAG welding gun which comprises a gun barrel. The rear end of the gun barrel is in threaded connection with two connectors. Two short wire feeding hoses are plugged into openings of the connectors. The gun barrel is wrapped with an insulating rubber sleeve as a whole. The front end of the gun barrel is fixedly connected with a gathering nut which is connected with a transition nut in a threaded mode. The transition nut is connected with a twin-wire welding nozzle through a locking nut. The front end of the gun barrel is plugged into the large open end of a jet nozzle, and the portion between the front end of the gun barrel and the twin-wire welding nozzle is covered with the jet nozzle. The multi-purpose single-contact-tube twin-wire MIG/MAG welding gun is simple in design, compact in structure and stable and reliable in performance, and can be used on MIG/MAG welding occasions needing a large welding condition span by cooperating with a matched single-power twin-wire-feeder mechanism, for example, quick molding in a welding mode; matched twin-wire contact tubes of different diameters can be applied through replacement, and the welding gun can also be applied to conventional occasions needing a high deposition rate.