Inconel 718 alloy samples with different densities were prepared by selective laser melting technology. The influence of processing parameters including laser power and scanning speed on the density of alloy samples was studied. The formation causes of micro pore defects and its effect on the tensile property were analyzed. The mechanical properties of alloys with different densities were studied by SIDA heat treatmet. The results showed that the interaction mode between laser and powder was determined by processing parameters. The "keyhole" mode occurred under the condition of high laser power and low scanning speed, which resulted in the decrease of density. When the power decreased or the scanning speed increased, the "keyhole" mode would change to the "heat conduction" mode, and the density would increase. However, when the laser power was too small or the scanning speed was too large, the unfused pore defects would occur, which would greatly reduce the density of the sample. The strength of the selective laser melting Inconel 718 alloy sample did not increase in a strictly monotonic manner with increasing the density. The morphology, quantity and size of the micro pore defects also had an effect on the tensile properties of the samples. SIDA heat treatment can greatly improve the microhardness and tensile strength of selective laser melting formed Inconel 718 alloy, but the plasticity is significantly reduced.