Laser Powder Bed Fusion (LPBF) produces parts with complex geometries that are typically difficult to create using the conventional manufacturing techniques. The complex phenomena related to material melting and solidification present great challenges for both experimental and numerical tools to investigate and to control the print quality. As a Lagrangian and particle-based discretization technique, the smoothed particle hydrodynamics (SPH) method [1] provides distinct advantages in handling large deformations and capturing moving interfaces encountered in LPBF simulations [2]. In this work, we develop a particle based SPH solver to simulate the LPBF process and test its validity by comparing the simulation results with experimental data concerning melt pool size. Subsequently, by combining the Synchrotron X-ray observations with high-fidelity simulations, we characterize the evolution of melt pools in situations with varying laser power, laser speed and laser spot size, while explaining the formation of either deep or shallow melt pools. The initial development of the melt pool under the influence of a laser heat source is further studied, with a close analysis of the factors affecting the formation of the melt pool. Finally, some parameter windows are obtained, which is of great helpful to improve the manufacturing quality.