Almost all the researchers devoted themselves to the ultimate strength of ship girder in static method previously. Ships sailing in the waves are always in motion, so that the hull structures are subjected to dynamic loads such as dynamic wave bending moment and slamming loads and generate dynamic response. The ultimate strength of structures under dynamic loads can reflect the safety level of ships more truly. Based on the Idealized Structural Unit Method (ISUM) and the solution method of dynamic equation, this paper develops the ISUM and applies it to the analysis of dynamic ultimate strength of stiffened plate. The accuracy and efficiency of ISUM are verified by comparing the calculation results with FEM, and the difference characteristics of structural strength under dynamic and static loads are analyzed. The influences of strain rate effect, damping and loading period on the ultimate strength of stiffened plate are discussed. This analysis is of reference significance for the safety and reliability design of ship structure.