For metal-inert gas welding (MIG), the welding heat input includes the arc heat flux on the surface of workpiece and the volumetric distribution of droplets' heat content within the weldpool. Based on the general principle of arc physics and the physical nature of the action of droplets and weldpool, a distribution mode of current density on the deformed weldpool surface and the heat content of filler metal droplets inside the weldpool is established in this paper. 7Numerical analysis is used to study the distribution of current density in MIG welding, the distribution mode of the heat content of filler metal droplets, weldpool geometry, the fluid flow and temperature profiles. Experiments are performed to prove the reliability of the calculated mode.