Gas atomization is a widely used method to produce metal powders. However, the powders often have a wide range of particle sizes, which is not ideal for additive manufacturing. To visualize the liquid breakup during gas atomization, we have developed a novel approach that combines similarity theory with Navier-Stokes equations. The simulation utilizes the volume of fluid to discrete phase model, which captures both primary and secondary breakups. By analyzing the morphology of the lumps and particles produced by a closed-coupled atomizer, the results have shown that approximately 30% of the lumps formed turn into fibers, and 68% secondary breakup mode is vibration breakup. Based on these simulations, we propose optimized methods for modifying the liquid outlet and adjusting the gas inlet eccentricity. Overall, our approach provides a new way to directly simulate the entire gas atomization process and offers valuable guidance for gas atomization technology.