During a loss of vacuum accident(LOVA), the air ingress into a vacuum vessel(VV) may lead to radioactive dust resuspension, migration, and even explosion, thereby posing a great threat to the safe operation of future fusion reactors; thus, it is crucial to understand the flow characteristics and radioactive dust transport behavior induced by LOVA. However, only a few studies have identified the characteristics of the highly under-expanded jet flow at a scale of milliseconds during LOVA. Particularly, the occurrence and behavior of a Mach disk is yet to be captured in existing studies. In this study, we used a more advanced model with a finer mesh and adaptive mesh strategies to capture the Mach disk in a VV during LOVA.In detail, a computational fluid dynamics–discrete phase model one-way coupled multiphase approach was established using the computational fluid dynamics code ANSYS FLUENT and applied to the analysis during the first seconds of LOVA. The results showed that air ingress into the VV behaved like a highly free under-expanded jet at the initial stage and Mach disk was formed at ～6 ms.Moreover, the flow field dramatically changed at the position of the Mach disk. The jet core before the Mach disk had a maximum velocity of ～8 Mach with the corresponding lowest static pressure(～100 Pa) and temperature(few tens of K). The friction velocities in the lower part of the VV, which is an area of concern due to dust deposition, were generally larger than 15 m/s near the inlet region. Lastly, the crude prediction of the particle trajectories demonstrated the spiral trajectories of the dust following the air motion. Therefore, this study provided a basis for further safety analysis and accident prevention related to dust transport and explosion in future fusion reactors.