Numerical simulation of additive manufacturing processes is time-consuming due to couplings of multiphysics phenomena. Addressing this challenge, efficient macroscopic and mesoscopic numerical methods have been introduced to mitigate computational costs. In macroscopic simulations, a steady-state thermal model based on new dedicated boundary conditions has been developed for fast predicting melting pool sizes. The proposed macroscopic model has been validated through the comparisons with transient model and experimental results. To investigate the effects of fluid dynamics, a 3D mesoscopic model based on finite element method has been proposed for simulating the multiphysics phenomena involved in additive manufacturing. The Arbitrary Lagrangian Eulerian (ALE) method is employed to simulate the evolution of the melt pool. Applications in additive manufacturing shows their efficiency.