For selective laser melting (SLM) forming of Ti6Al4V, a three-dimensional (3D) mesoscopic model of random distribution of powder particles was established based on the discrete element method (DEM). The volume of fluid method (VOF) was used to track the 3D dynamic free surface in SLM forming process. Various factors were considered in the numerical model, such as the TC4 powder bed with randomly distributed particles, the thermophysical parameters changing nonlinearly with temperature, the free surface evolution of the molten pool, the surface tension caused by temperature gradients, and the evaporation effect. The heat transfer, melting, flow and solidification in the interaction between laser and powder particles were studied according to the numerical simulation. Results show that the Marangoni convection induced by temperature gradient and surface tension gradient is the main factor affecting the heat and mass transfer within the melt pool and the 3D morphology of the melt pool. The line energy density (LED) is positively correlated with the Marangoni effect. The quality of single track surface was good when the optimized LED ranged from 92.9 J/m to 183.0 J/m. The three-dimensional size and the morphology of the molten pool and the molten track were observed and analyzed by the single track forming experiment, by which the numerical results were validated.