The mechanisms of the internal ballistic process and the evolution of the precursor flow field within an underwater submerged launch are investigated. A bidirectionally coupled computational method is proposed,wherein the fourth-order Runge–Kutta method is employed by external programs to solve the interior ballistic model,providing accurate bottom pressures of the projectile. Simultaneously,the Computational Fluid Dynamics (CFD) method is applied to solve the fluid model,offering precise front pressures of the projectile. Ultimately,the state of the projectile’s motion along the barrel is determined. A velocity–pressure separation solution algorithm,Semi-Implicit Method for Pressure Linked Equations (SIMPLE) and the Schnerr–Sauer cavitation model,is utilized to solve the Volume of Fluid (VOF) multiphase Navier–Stokes equations with compressible cavitation. The interior ballistic process with a cavitation reaction of the water column flow ahead of the projectile in the barrel is simulated and verified by underwater launch experiments. The development of the precursor flow generated by the precursor fluid is simulated,and the initial flow field at the muzzle is obtained. The results demonstrate that the projectile will experience an acceleration–deceleration–acceleration motion process due to the change in the projectile drag. The compression and expansion waves of the underwater internal ballistic process are the main cause of the large variation in drag. Furthermore,the cavitation cavity strength formed by the precursor flow has a tendency to increase and then decrease.