[Display omitted] • Close-coupled nozzles for gas atomization with different gas Mach number at the outlet were designed. • The structure of gas field and the profile of gas pressure and velocity were analysed by CFD. • The cooling and solidification process of metal droplets were analysed calculated by finite difference method. • Fe-6.5wt.%Si powder was prepared and close-coupled nozzles with higher Mach number can produce finer powder. Gas velocity is a key parameter regulating the particle size and the cooling rate of the gas atomized powder applied in additive manufacturing, metal injection molding, thermal spraying, and soft magnetic composites. In this paper, on basis of the well-designed close-coupled nozzles with different gas Mach numbers at the outlet, the gas field structure was simulated by Computational Fluid Dynamics (CFD) software, and the process of cooling and solidification of Fe-6.5 wt% Si metal droplets was calculated by finite difference method. The results show that with the increase of Mach number, both the gas velocity downstream and the pressure at the base of melt delivery tube tip rise, whereas the mass flow rate of the melt decreases. The nozzles with high Mach number can produce finer powder with higher cooling rate. The median diameter of the powder prepared by the nozzle with Mach numbers of 1.0, 1.5, 2.0, and 2.5 is 44.9, 39.0, 32.5, and 29.1 μm, respectively, and the corresponding cooling rate of the metal droplet with a diameter of 80 μm is 2.85 × 104, 2.98 × 104, 3.32 × 104, and 3.50 × 104 K/s, respectively. This work provides new ideas and suggestions for the preparation of metal powder with small particle size at high cooling rate. [ABSTRACT FROM AUTHOR]