To obtain the change of thermal stress in the slow cook-off test of explosive charge, a set of thermal stress testing device was designed.The change curves of the thermal stress with temperature of octogen(HMX) based aluminized explosive charge under the closed confinement condition were obtained.Through adjusting the volume ratio of inert coating layer and charge, and changing the rising rate of thermal stress in the slow cook-off process, the critical response temperature of cook-off bomb and the intensity of response under three working conditions were researched.The difference of change rate of thermal stress, the whose process can be divided into six stages.Its influencing factors is thermal expansion, porosity decrease, HMX crystal transformation, slow decomposition of HMX, partial small molecule gas leakage, and accelerated decomposition of HMX in the order.When the temperature rises to 208 ℃, the thermal stress reaches 9.2 MPa and the charge is ignited immediately.Increasing the silicone rubber coating layer with greater thermal expansion on the charge surface of cook-off bomb, can make the growth rate of the thermal stress of charge accelerate and the critical ignition temperature of charge reduce, but the drastic degree of the response of charge does not change.