To study the melt pool dynamics and resultant porosity formation mechanism of the selective laser melting (SLM) Inconel 625 alloy, a height function-lattice Boltzmann method (HF-LBM) coupled model that considers both the computational efficiency and the important physics was proposed. In this model, a novel interface captured technique was used to provide insights to simulate the melt flow by considering interfacial forces via surface tension, Marangoni convection and recoil pressure. It was discovered that the fused powders could form a continuous cladding layer under the effect of surface tension rather than gravity, as the strength of surface tension was a million times larger than that of gravity. And the visible elevation at the beginning of the melt track was strongly associated with the recoil pressure. The simulation results matched well with the experimental observations. The porosity formation mechanisms under different process parameters and powder packing densities have also been investigated. The results revealed that at a fixed laser power (120 W), the melt track exhibited with porosity in the form of a lack of fusion when the scanning speed was above 1200 mm/s and the discontinuous melt track was thought to yield subsequent porosity due to the elevated layer thickness. In contrast, the porosity displayed in the form of trapped gas owing to the keyhole effect under high energy input. When a low powder packing density was applied, the "necking" phenomenon, attributed to the persistent flow rate difference between the bottom and the rear, was observed and thought to increase the porosity similar to the situation of high scanning speed. (C) 2019 Elsevier Ltd. All rights reserved.