This paper establishes a new similarity method for the problem of truncated ogive projectiles penetrating ship plates. This method can consider plates made of different materials and summarizes the relationship between material strength and target plate thickness through an energy-based formulation and dimensional analysis. Using the finite element software DYNA, multiple sets of numerical simulations are carried out, and the power parameter of the target plate thickness is determined. Subsequently, the scope of the penetration problem is extended to the stiffened plate structure of a ship. When a stiffener is considered, the change in the attitude angle of the projectile is very important. The deflection of the projectile may lead to incomplete penetration of the target plate, or even cause the projectile to ricochet. The prototype response can be accurately predicted by compensating all structural dimensions of the stiffened plate according to the plate thickness correction parameters. On this basis, adjusting the ventral plate thickness of the stiffener effectively changes the trend in the attitude angle of the projectile. The errors in the residual velocity and attitude angle of the projectile are guaranteed to be within a reasonable range.