To study the dynamic response characteristics and transient multiphase flow field evolution during the underwater navigation of high-speed projectiles, a two-dimensional unsteady multiphase flow model for underwater sealed launch was established. The model employs a VOF multiphase flow method coupled with the Schnerr-Sauer cavitation model to track multiphase interfaces and incorporates the Standard k-ε turbulence model to describe the gas-liquid turbulence mixing. The feasibility of the model was validated through underwater launch visualization experiments. On this basis, the entire launch process of a 30mm underwater smoothbore gun at varying depths was numerically analyzed by UDF coupled the interior ballistic model and overset mesh technology. The results show that increasing launch depth slightly reduces muzzle velocity while elevating chamber pressure; After the projectile exits the muzzle, higher pressures in deeper water environment cause propellant gas to expand axially, resulting in greater thrust during the intermediate ballistic phase. Across three launch depths, stable cavitation bubbles form on the flank of the projectiles during underwater navigation, with high-stress zones at the warhead causing projectile velocity to decay linearly over time.