The metal powder size distribution is mainly affected by the metal droplet secondary breakup process in gas atomization. In the present work, a two-dimensional axisymmetric mathematical model, which takes a secondary breakup and solidification model into consideration, is developed to describe the atomizing gas flow features, including the velocity, pressure, and temperature distribution, and the metal droplet breakup process. The results show that the high-pressure gas stream forms a supersonic jet, which rapidly expands in the radial and axial direction after exiting the nozzle. In addition, an inverted conical recirculation zone forms below the feeding tube. As the gas moves, the metal droplets gradually break up into small-sized droplets. Although the size of metal droplet significantly decreases in the early stage of breakup process, the temperature and velocity of the metal droplet almost remain stable. The metal droplet temperature reduces sharply and its velocity increases rapidly, only when the droplet size becomes small, followed by the solidification process. At the outlet, the metal droplets that initially had a size in the millimeter range break up into micronsized droplets. In addition, the median diameter of metal droplet at the outlet is similar to 51.0 mu m, which is basically consistent with the production data. Furthermore, the effect of the initial metal droplet diameter on the metal droplet breakup dynamics is also investigated. With the reducing initial metal droplet diameter, the size distribution interval of the metal droplet narrows down and the median diameter of metal droplet decreases significantly.