Gas metal arc additive manufacturing (GMAAM) is one type of additive manufacturing, is the process of melting the metal wires by the arc and building the parts layer by layer. It has many advantages such as low cost, high deposition rate and capability of producing large-scale parts. However, the high energy source arc normally introduces over-heating issues, which usually increase the operational challenges as well as deteriorate the performance of additive manufacture metals parts. GMAAM based on controlled cooperatively by main and auxiliary wires (M/AWAAM) is a novel designed processing by using a system of narrow gap arc welding, aiming to address the over-heating issues and improve the welding efficiency and quality. The application of this single arc welding system with coordinated control of main and auxiliary wires to additive manufacturing can solve a series of problems such as poor process stability, severe heat input, and serious heat accumulation during GMAAM, and greatly improve arc additive efficiency. This project will focus on understanding the mechanisms of how M/AWAAM tailors the microstructure and improves the properties of developed parts by integrating experiments with computer modelling. Undoubtedly, M/AWAAM can take maximum advantage of the synergies between main and auxiliary wires in arc additive manufacturing. It can realise high-efficiency, high-quality and low-cost additive manufacturing process through a combination of controllable arc input energy, fast additive speed and excellent joint performance. Thus, it can meet the urgent needs of large-scale complex precision components in the industrial sectors of aerospace, automotive, defence, nuclear and energy.