The invention aims at providing a plasma diversion fusion electrode arc welding device and welding method. When fusion electrode plasma arc welding is conducted, a plasma welding gun jet nozzle is used as a bypass to distribute a part of welding current passing through a base material. In this way, large current can pass through a fusion electrode welding wire. While the high deposition rate and stable globular transfer are realized, input of heat acting on the base material is reduced, and the welding quality is improved. The plasma jet nozzle is connected with a cathode dividing electric arcs, burning damage to the jet nozzle can be greatly reduced only through part of current, the bearing capacity of the welding gun is improved, and the welding process is stabilized. Through combinationwith the advantages of the low plasma diversion heat input, the high fusion electrode plasma arc melting efficiency and the stable welding process, the filling welding wire can be heated at high efficiency, and the stable welding process and the good welding quality can be ensured. The welding method is suitable for multi-layer and multi-pass welding of thick plates and high-speed welding of thinplates.