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In order to study the influence of WC addition on the microstructure evolution and use security of selective laser melted metal, economical spherical WC powders produced by granulation-sintering-deoxygenation (GSD) method were chosen as reinforcements to manufacture powder reinforced 18Ni300 steel matrix composites via selective laser melting under the given forming parameters after powder mixtures in this paper. The effects of reinforcement contents on the microstructures and impact properties were analyzed. The results reveal that the WC powder contents are the key factor of determining relative density and impact toughness of the composites. The relative density of the composite has few changes, but the dense composites with no defects can not be manufactured when the WC content is higher than 25% under the given forming parameters. Under the forming parameter, metallurgical bonding can be formed between WC powders and the matrix. The W and C elements in the matrix of the composites increase due to partial fusion of WC particles. α-Fe phase formation is restrained, and the grain size and orientation change. With increasing WC contents, α-Fe phase gradually transforms into γ-Fe phase, and grain sizes increase. The average impact energy of the samples decreases from 49.25 J to 8.5 J with WC weight contents increasing form 0% to 20%. The SLMed 18Ni300 exhibit plastic fracture characterization, and the fracture characterization gradually transforms form the mixture of plastic and brittle fracture characterization to brittle fracture characterization.