Abstract: It’s one of the more feasible solutions for precision machining in the future that a mount of the workpieces produced by the additive manufacturing technology are connected by welding. Reduced activation ferrite/martensitic (RAFM) steel plates produced with four different particle sizes powder (0 ~ 25, 15 ~ 53, 45 ~ 105, > 100 μm) additive manufacturing are connected by laser welding technology. The microscopic microstructure evolution characteristics of laser welding joints were characterized. The results show that the unfused defects of additive manufacturing RAFM steel with powder particle size less than 25 μm are repaired in the weld area, while the unfused defects of the heat-affected zone and the base metal area cannot be improved. The defects in the welding of other steel plates are mainly pores, which are both distributed in the weld area and the base material area. And the number of steel welding pores with powder particle size exceeds 100 μm is much greater than other two welds. The microscopic structure of the weld area of the four joints is a coarse slatted martensite, and the columnar crystals grow from the edge of the pool to intersect the centerline. The characteristics of the additive manufacturing process lead to precipitation zone in heat affected area and base metal zone. The peak temperature of the quenched zone near the weld is higher, which is fine martensite structure. The tempering area far away from the weld is composed of secondary tempered pearlite structure, and therein grains partially get coarsened.