Dynamic characteristics of the breech mechanism directly affect warship’s reliability. Most of the research on the breech mechanism is related to locking chamber and firing primer, while few articles are found out about extracting cartridge-case. In this paper, nonlinear finite element method is used to research on flexible body dynamics of the breech mechanism. At first, structure and cartridge-case extracting process is analyzed. Then stresses and case speeds caused during the extracting process are tested. Flexible body dynamics model of some breech mechanism is built by using PROE, Hypermesh and ABAQUS software. Three models are used to compared with the experiment, which include three different element types. Finally, kinetic energy, internal energy, extractor stresses and case average speed are analyzed after wear of the extracting template.