Through the development in past decades, the capability of metal additive manufacturing (AM) has been extended from single-material to multi-material fabrication, which enables the in-situ element tailoring and user-defined functionality. Despite the attractive capability, the defects caused by the multi-phase and multi-material interactions in the metal fusion process severely degrade the mechanical properties, reproducibility and reliability of as-built parts, which hinder the extensive industrialization of multi-material AM technologies. In this work, we developed the meso-scale simulation models to study the process dynamics in multi-material AM. In the first part, we focus on the AM of particle-reinforced metal matrix composite (MMC), where the reinforcing particles remain unmelted in the metal fusion process. We propose a Computational Fluid Dynamics and Discrete Element Method (CFD-DEM) model to reveal the dynamics of molten pool and reinforcing solid particles during the AM process. The results demonstrate that the interface effect and the molten pool flow play significant roles in the dynamics of reinforcing solid particles. Based on the understanding of reinforcing particle dynamics in the AM process, some practical fabrication strategies are proposed to eliminate the particle agglomeration in the as-built part, which shows the capability of AM to achieve spontaneous dispersion of reinforcing solid particles in the metal matrix. In the second part, we focus on the AM of in-situ alloying components, where different kinds of metal feedstocks are simultaneously melted in the molten pool. We propose a Computational Fluid Dynamics and Calculation Phase Diagram (CFD-CALPHAD) model to reveal the thermal effect of chemical reaction during the metal alloying process. The results demonstrate that the composition-dependent melting point can significantly influences the macroscopic segregation pattern at the melt track. Moreover, the enthalpy of mixing can remarkably enlarge the molten pool size through the positive feedback loop of free surface temperature, which induces the drastic change of molten pool dimension with the energy input. Through controlling the deposition sequence of different metal feedstocks, the positive feedback loop of free surface temperature can be transferred into negative feedback loop to restrict the molten pool dimension, which not only extends the process window but also improves the energy efficiency in AM process.