In the selective laser melting process, the molten pool flow, particularly the keyhole phenomenon, plays an important role in the pore formation and then influences the mechanical properties. In this study, we develop a 3D numerical model to simulate the keyhole mode in SLM. The transient evolutions of the temperature field, fluid flow, molten pool geometry and energy absorption have all been reproduced. The simulated molten pool dimensions agree well with the experimental data, including the fluctuation. More thorough insights into the underlying physical mechanisms are achieved, including the correlation between the keyhole geometry and laser absorption, the influence of manufacturing parameters on the melting mode, and the fluid flow pattern under different melting modes. These understandings can provide guidance for the optimization of manufacturing process parameters to improve the product quality.