Laser melting deposition (LMD) is a promising way for the fabrication of complex reduced activation ferritic/martensitic (RAFM) steel components which provides an exceptional opportunity to improve the existing designs and move toward fabricating fine features and complex geometries with higher efficiencies. It is well known that if steel members are welded together, defects will be concentrated in the welded joint. We can control the scanning path of the laser to achieve a complex component molding, avoiding welding. Considering the LMD technology, the results of each layer of powder molding will affect the fineness of the final processed products, such as laser power, scanning speed, defocus and so on, which are all important factors affecting the products. We control the size of powder particle size as a variable to conduct a study on RAFM steel supplemented by LMD technology for the first time, and its microstructure and grain size were analyzed. In terms of mechanical properties, its hardness, Charpy impact, tensile strength and elongation were measured. The results show that compared with other processing processes including Selective Laser Melting (SLM), rolling, welding joints, the tensile strength of RAFM steel manufactured by laser coaxial powder feeding is up to 1057.75 MPa, and there are no pores and almost no defects when using small particle size powder. The microstructure of LMD RAFM steel contains a large amount of lath martensite and δ-ferrite, and some precipitates such as Mx, whose average grain size decreases with the increase of powder particle size (5–150 μm). In this study, the laser additive process was monitored by high-speed photography, and the average grain size of LMD RAFM steel was further analyzed, and a reasonable explanation of its mechanical properties was given.