<p>Vacuum casting, as a secondary metallurgical refining technology, demonstrates significant advantages in enhancing ingot purity. During this process, degassing predominantly occurs in the secondary fragmentation stage of the molten steel. The specific surface area and vacuum degree of droplets are the main factors determining the degassing efficiency. This study established a discrete continuous phase-coupled model for molten steel atomization and systematically analyzed the atomization behavior under varying degrees of vacuum. The results indicate that when the vacuum degree drops below 60 Pa, the changes in atomization angle and droplet size can be ignored. Through studying the vacuum degree’s effect on degassing time for gases (nitrogen, hydrogen, oxygen), deoxidation was identified as the governing factor in degassing efficiency. Linear regression analysis revealed that above 60 Pa, specific surface area governs degassing efficiency. Below 40 Pa, however, the pressure difference becomes the dominant factor, superseding the effects of surface area. Considering both degassing efficiency and production economics, 60 Pa is established as the critical vacuum degree that optimizes the cost-benefit synergy for this process.</p>