Abstract: Magnetically controlled plasma-FCAW hybrid welding technology is proposed as a new efficient underwater welding method. The advantages of the two independent welding processes are complementary through the unique welding torch structure design. An external magnetic field was designed by regulating the coupling degree between two arcs for the problem of arc repulsion caused by different polarity of power supply in hybrid welding process to solve. The influence of main process parameters on the weld forming and section geometry characteristics of Q355B steel was studied. The results show that the external magnetic field can effectively improve the stability of the hybrid welding process and weld forming. FCAW voltage greatly influences the stability of the underwater hybrid welding process. Plasma welding current and FCAW voltage greatly influence weld penetration, and the plasma current has an approximately linear relationship with weld penetration. Compared with the underwater FCAW process, the welding depth of the hybrid welding process is increased by more than 40%, with higher welding efficiency and welding stability.