Abstract: When immersion heaters were electrified overheating, secondary recrystallization could be observed on cannula at the partition with heating cord in it, but could not be observed at the partition without heating cord in it. A new material evidence identification method, by comparing the grain size and microstructure state of cannula at different partition, was proposed to certify whether the immersion heaters were electrified overheating or not. The results of simulation experiments and theory analysis show that it’s feasible to identify whether the fire hazard was caused by electrified overheating of immersion heater or not.