Abstract: Butt welding of 12 mm thick Q345 marine steel is carried out by using the bypass coupling triple-wire indirect arc (BCTW-GIA) welding and metal active gas(MAG) welding. The welding energy consumption, efficiency, mechanical properties and joint structure of the two welding processes are compared and analyzed. The results show that under the conditions of the total welding current of 420 A and welding speed of 600 mm/min, the single-pass weld with good formation is obtained by the BCTW-GIA welding, and the average tensile strength of the welded joint is 505 MPa. However, the MAG welding achieves good welding formation with a total welding current of 700 A and a three-layer and three-pass welding process, and the average tensile strength of the joint is 534 MPa. Under the experimental conditions, the welding energy consumption of the BCTW-GIA welding is only 63% of that of the MAG welding, and the welding wire deposition rate is 2.76 times of that of the MAG welding.