Simulation models can be effectively used to predict welding distortion and residual stress in laser welding systems. To this end, developing a sophisticated heat source model for finite element analysis (FEA) is crucial. This study proposes a novel multi-layered heat source model for FEA to predict the melting zone of the welds in fiber laser welding. The proposed model has multiple degrees of freedom sufficient to simulate various heat sources. A systematic approach by solving inverse problems with global optimization is devised to estimate heat source model parameters. To achieve this, a simplified FEA model is incorporated to accelerate the simulation process. Experimental results using 9% nickel steel indicate that the proposed framework provides more accurate predictions on the melting zone of the welds than existing methods while reducing the computational cost by 1/2000. The proposed framework can be used by field engineers to find the relevant heat source subjected to on-site welding conditions, minimizing trials and errors. With this method, fast and accurate welding simulations can be performed, ultimately leading to improved productivity.