Abstract: During the underwater multiple projectiles launch process, the projectiles operate in a complex and dynamic flow field environment. The trajectory deflection of projectile is influenced not only by initial conditions such as velocity and crossflow but also by mutual interference effects among multiple projectiles. To investigate the cavitation evolution and trajectory interference characteristics in multiple projectiles underwater launch, this study establishes a numerical simulation model based on the overlapping grid technique and finite volume method, coupled with a six-degree-of-freedom (6-DOF) motion model. The influence mechanisms of spatial arrangement mode, launch velocity, and crossflow on trajectory deflection are systematically analyzed. The results demonstrate that: (1) The spatial arrangement has a minor impact on trajectory deflection and an equilateral triangular configuration can be adopted in practical applications to optimize launch space utilization; (2) As the launch velocity increases, the wake interference between projectiles intensifies, leading to significant flow field disturbances and stronger mutual trajectory interference; (3) Higher crossflow velocities exacerbate asymmetric cavitation development near the projectile shoulders, and when the crossflow exceeds 0.75m/s, it becomes the dominant factor in trajectory deflection. These research findings provide a theoretical basis for trajectory prediction and layout optimization in multiple projectiles underwater launch.