According to IEC61643.1,the thermal stability experiment based on stepping current should be carried out to test the thermal stability characteristics of Surge Protective Devices (SPD).However,we found some deficiencies in the thermal stability experiment based on stepping current through practical tests.Firstly,the actual degradation of Metal Oxide Varistor (MOV) cannot be exactly simulated in laboratory environment,therefore the simulated thermal stability environment is not sufficient for actual thermal conditions.Secondly,the MOV sensitivity to voltage varied under different voltages.When voltage increased from 640 V to 680 V,the breakdown time changed by 68.3%,while the current changed by 289.3%.Thirdly,the experiment is required to be carried out under a constant current within only ±10% fluctuation,which is unavailable due to the limited sensitivity of test instruments.The above deficiencies,found in thermal stability experiment based on stepping current,are believed to be of engineering values for improvement in thermal stability measuring technology for SPD.