The relationship between slow cook-off behaviour and thermal decomposition characteristics of NEPE propellant and HTPB propellant was analyzed by simultaneous differential scanning calorimetry and thermogravimetry (DSC-TG), scanning electronic microscope（SEM）and slow cook-off test. A porous state generated in the decomposition of ammonium perchlorate（AP）at low temperature is the main reason causing HTPB propellant reacting violently in the slow cook-off test. The energy released by nitrates at lower temperature enables NEPE propellant to ignite before AP and HMX begin to decompose, which show that NEPE propellant has a milder reaction in slow cook-off test.