Abstract: To explore the mechanisms of moderately thick water-backed metal plates obliquely penetrated by high-velocity blunt-nosed projectiles, the oblique penetration was converted to corresponding perpendicular penetration through thickness equivalence. The conversion was conducted according to different mechanical states and energy-dissipative mechanisms during the oblique penetrating process together with the high-velocity oblique penetration-resistant characteristics of moderately thick water-backed metal plates. Then, the whole process was divided into three consecutive phases, i.e., the compression & mushrooming phase, shearing & compression phase and shearing & intruding phase. Based on the three-phase penetration mechanism, an analytical model was established to calculate instantaneous residual velocities of the blunt-nosed projectiles obliquely perforating moderately thick water-backed metal plates. Moreover, the limitations of the present model were discussed. By adopting the present model, the instantaneous residual velocities of 3.3g cubic projectiles obliquely penetrating 5mm-thick water-backed steel plates have been calculated. Good levels of agreement were obtained between the theoretical values and experimental results as well as corresponding numerical results. Considering several phenomena, such as the dynamic supporting action and kinetic energy dissipation effort of the water medium behind the target, the three-phase model can be employed to reasonably predict, in the valid regime, the instantaneous residual velocities of the blunt-nosed projectiles obliquely perforating moderately thick water-backed metal plates, and therefore, has theoretical and engineering application values.