Laser Powder Bed Fusion is a mainstream Additive Manufacturing process for metals. The process is dynamics with diverse physics involved in a fully coupled manner. To quantitatively capture the dynamic phenomena in the process, a multi-physics model has been established. A Computational Fluid Dynamics (CFD) module is included to concurrently treat the compressible/incompressible flows in the gas region and the melt pool, and a Computational Particle Dynamics module based on the Discrete Element Method is included to track the powder motion as driven by the gas-powder and powder-powder interaction forces. The model predictions of keyhole dimension and powder speed are found to be consistent with the measurements via the synchrotron X-ray imaging results. The simulation results of the laser absorption, mechanical driving forces, vapor gas flow structure, and powder status are further leveraged to understand the complex physics during the process.