Abstract: Abstract: High-intensity impact loads experienced by a projectile during high-speed water entry can induce pronounced dynamic structural responses and deformation, and may further lead to structural damage or failure. Therefore, investigating this process is of great significance. To reveal the fluid–structure interaction characteristics and the deformation mechanism of the tail structure of a projectile equipped with a tail fairing, a numerical simulation based on the arbitrary Lagrangian–Eulerian (ALE) method was conducted for its water-entry process at a velocity of 40 m/s. The reliability and accuracy of the numerical model were validated through comparisons with experimental results and grid-independence verification. In the model, the water and air domains were described using an Eulerian formulation, while the projectile structure was modeled using a Lagrangian formulation. The compressibility of water and air was considered through corresponding equations of state, and the projectile material was defined using an elastoplastic constitutive model. The fluid–structure interaction between the projectile and the surrounding fluid was realized using a penalty-based coupling algorithm. To improve the stability and reliability of the coupling calculation, different penalty factors were compared, and the acceleration response and sliding interface energy were analyzed to determine an appropriate coupling parameter. A three-dimensional computational domain containing both water and air was established, and local mesh refinement was applied to the impact region and key structural regions of the tail fairing. The reliability of the numerical model was validated by comparing the simulated cavity morphology, projectile displacement, and projectile velocity with experimental results. Meanwhile, a grid-independence study was conducted to balance computational accuracy and efficiency. Based on the validated numerical model, the effects of entry angle and angle of attack on cavity evolution, impact load, stress-wave transmission, and deformation of the tail fairing were systematically investigated. The results indicate that, as the entry angle decreases, the cavity becomes increasingly asymmetric, accompanied by more complex free-surface deformation and cavitation phenomena, while the overall impact load on the projectile decreases accordingly. The deformation of the tail fairing is jointly governed by stress-wave propagation and cavitation effects, exhibiting a non-monotonic dependence on the entry angle: it first decreases and then increases, reaching a minimum at an entry angle of 60°. For oblique entry with a positive angle of attack, the cavity asymmetry is stronger than that under a negative angle of attack, and the peak acceleration decreases with increasing angle of attack. Under negative angles of attack, the peak acceleration first decreases and then increases as the absolute value of the angle of attack grows. A sufficiently large angle of attack can trigger secondary tail re-entry, producing a “tail-slap effect” and a second acceleration peak, which is more pronounced for positive angles of attack. In addition, a positive angle of attack markedly increases the deformation of the tail fairing, whereas a small negative angle of attack can reduce the deformation; however, the deformation increases again as the magnitude of the negative angle of attack further increases.