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Mg–3Nd–0.2Zn–0.4Zr (NZ30K, wt.%) alloy is a new kind of high-performance metallic biomaterial. The combination of the NZ30K Magnesium (Mg) alloy and selective laser melting (SLM) process seems to be an ideal solution to produce porous Mg degradable implants. However, the microstructure evolution and mechanical properties of the SLMed NZ30K Mg alloy were not yet studied systematically. Therefore, the fabrication defects, microstructure, and mechanical properties of the SLMed NZ30K alloy under different processing parameters were investigated. The results show that there are two types of fabrication defects in the SLMed NZ30K alloy, gas pores and unfused defects. With the increase of the laser energy density, the porosity sharply decreases to the minimum first and then slightly increases. The minimum porosity is 0.49 ± 0.18%. While the microstructure varies from the large grains with lamellar structure inside under low laser energy density, to the large grains with lamellar structure inside & the equiaxed grains & the columnar grains under middle laser energy density, and further to the fine equiaxed grains & the columnar grains under high laser energy density. The lamellar structure in the large grain is a newly observed microstructure for the NZ30K Mg alloy. Higher laser energy density leads to finer grains, which enhance all the yield strength (YS), ultimate tensile strength (UTS) and elongation, and the best comprehensive mechanical properties obtained are YS of 266 ± 2.1 MPa, UTS of 296 ± 5.2 MPa, with an elongation of 4.9 ± 0.68%. The SLMed NZ30K Mg alloy with a bimodal-grained structure consisting of fine equiaxed grains and coarser columnar grains has better elongation and a yield drop phenomenon.