Abstract: The effects of jet angle (40°, 50°, 60°, 70°, 80°), jet width (15,20,25,30,35 mm), and diversion pipe extension length (0.5, 4.5, 4.5, 8.5 mm) of Laval nozzle used for close-coupled gas atomization on flow velocity of flow field, flow velocity at the flow junction and the static pressure at the outlet of liquid pipe along y-axis in atomizing chamber were simulated by computational fluid dynamics (CFD). The optimized structure parameters of the nozzle were obtained. The 18Ni300 die steel close-coupled gas atomizated powders were prepared with the optimal-structured nozzle, and the performane of the powder was measured. The results show that with the increase of jet angle, the maximum flow velocity of the flow field decreased, and the flow velocity at the flow junction and the static pressure at the outlet of liquid pipe increased. With the increase of the diversion pipe extension length, the maximum flow velocity of the flow field decreased first and then increased, the static pressure at the outlet of liquid pipe decreased, and the flow velocity at the flow junction did not change significantly. With the increase of jet width, the maximum flow velocity of the flow field did not change significantly, and the flow velocity at the flow junction and static pressure at the outlet of liquid pipe decreased. According to the simulation results, the optimized structure parameters of nozzle were listed as follows: jet angle of 60°, jet width of 30 mm and diversion pipe extension length of 4.5 mm. In this case, the 18Ni300 die steel close-coupled gas atomizated powder had regular shapes and small and uniform size, the fine powder yield was the largest, and the powder flow rate and apparent density were relatively large. The comprehensive performance of the powder was the best.