In this paper, according to the structure of warships, the equivalence method of the protective structure with reinforcing ribs is presented abided by the principal of equivalence in rigidity. The minimized model and the equivalence model of the protective structure with reinforcing ribs penetrated by projectiles are investigated in experiment and numerical simulations, and the results show that the equivalence method of the protective structure is reasonable. The reinforcing ribs are extensively used in protective structures, especially in ship structures. The reinforcing ribs are characterized in extensive distribution range and little weight of unit length, and have the main effect that the rigidity requirement of the structure is satisfied so that the structure has enough rigidity stability. However, the big and the small reinforcing ribs in actual structures originally have small size of the cross sectional area (CSA). According to a usual model research method, after an original structure is proportionally reduced according to the minimized proportion of one to four , the size of the CSA of the big and the small reinforcing ribs becomes smaller, and the shape of the cross sectional area is shown in Figure1. Under such circumstances, an experimental research on the dynamic response of the reinforcing ribs under the impact load has the characteristics of great difficulty and bad feasibility, and in addition, the reliability of reflecting target characteristic by a proportionally reduced model is very low. In order to study the action of the reinforcing ribs more objectively, the T-shaped cross section of the reinforcing ribs is converted to a rectangular cross section according to the principle of rigidity equivalence, and then, the action can be researched actually. The study method is given as follows.