The competitive growth and the evolution of grain/dendrite structures of IN718 superalloy in the molten pool during selective laser melting (SLM) were investigated with the finite element method (FEM) coupling the phase-field model (PFM). The thermal evolution was solved by the FEM and then the results were input into the PFM model to simulate the microstructure evolution involving grain nucleation, grain epitaxial growth and grain competition. Based on the analysis of microstructure evolution of the transverse and longitudinal sections of the molten pool, the mechanism of dendrite competitive growth in the SLM process was discussed and the concentration distribution of the solute element Nb in the molten pool was quantitatively calculated. The results show that the dendrite competitive growth is affected by both the temperature gradient and the crystallographic orientation. The unfavorable orientation grain with a high misorientation (>22.5°) will be eliminated by the favorable orientation grain quickly.