In this manuscript, the effect of the heat treatment at 900 oC on the microstructure and passive behavior of 316L stainless steel fabricated by selective laser melting (SLM-316L ss) were investigated. In order to reduce the thermal residual stress and obtained excellent mechanical properties, the heat treatments at 900 oC were conducted on SLM-316L ss for 0, 0.5 h, 1.0 h, 3.0 h, and 5.0 h. Then, the microstructure, corrosion resistance, and passive film were characterized using SEM, TEM, EDS, XPS, and electrochemical measurement. Based on the results of microstructure analysis, it can be seen that when heated at 900 oC, the dislocations and sub-grain boundaries in the SLM-316L ss reduce until disappear with the heating time extending, accompanied by the precipitation of MnS inclusions, carbides and σ phases along the grain boundaries. The potentiodynamic polarization reveals that in the buffer solution with 0.1 M NaCl, pitting corrosion occurs on the samples with different heat treatments, and the sample with longer heating time possesses more negative pitting potential. According to EIS results, with the heating time extending, the diameters of incomplete arcs in the Nyquist diagrams decrease, and the calculated film thicknesses using the fitting results of the EIS data also reduce. The potentiostatic polarization under 0.1 VSCE, 0.2 VSCE, 0.3 VSCE, 0.4 VSCE, and 0.5 VSCE were utilized for the formation of passive films on SLM-316L ss with different heat treatments. Though fitting the Mott-Schottky curves, the negative slopes demonstrate that the formed passive films on the samples belong to the n-type semiconductor, and the calculated the densities of the point defects become larger in the sample with longer heating time. In addition, there is a logarithmic relationship between the carrier densities and the formation potentials of the passive films, and the calculated diffusion coefficient of point defects across the passive film of SLM-316L ss using the logarithmic relationship is larger when heated for a longer time. Based on the results of Mott-Schottky, a theoretical model related to the energy band structure and space charge layer is obtained to explain the electrochemical reaction on the passive film/solution interface. The XPS measurements confirm that the passive films formed on the SLM-316L ss are mainly instituted of oxides and hydroxides of Fe and Cr. In a summary, the heat treatments at 900 oC result in the reduce and disappearance of dislocations and sub-grain boundaries and the precipitation of MnS, carbides, and σ phases along the grain boundaries, which is responsible for the deterioration of the passive behavior of the SLM-316L ss.