To explore the design method of sealing structure which can meet the slow release requirements of warhead charge under slow cook-off condition，a small-sized cook-off bomb with adjustable connection strength and prefabricated area of pressure relief hole was designed. For octogen (HMX)-based aluminized explosive charge, the conditions of no more intense reaction occurs than combustion were obtained by experiments, including the strength threshold of shell and area threshold of pressure relief channel of cook-off bomb, etc. parameters. Based on this, a design method of two-stage sealing slow-release structure, which can satisfy the requirement of high connection strength and reliable pressure relief of the end cap, was proposed and the method was tested and verified in the cook-off bombs with kilogram charge. The results show that when two-stage sealing design for cook-off bomb is adopted, if the ratio of the area of the pressure relief channel to the charge cross section is not less than 30%, and the pressure of the gas product does not reach 96.5 MPa in the slow cook-off process, the pressure relief passage can be opened, which avoids the further growth of charge reaction, makes the combustion occur only and keeps the integrity of shell structure.