The fluid field of frangible cover opening by gas jet is complexly unsteady. In order to solve the problem caused by boundary changes of back cover after pressure arriving the opening condition, dynamic mesh was used to deal with its movement, and fragmental boundary conditions which can be modified one by one were used to simulate its break. Some different flow fields were analyzed with different boundary conditions. Comparison of the simulation results shows that the process of back cover opening which followed with break and movement is important to the wave's creation and transmission in launching canister.