Abstract: In this paper,simulating four conditions of the immersion heater:normal used,electrical superheated,heated in fire after normal used,and heated in fire after electrical superheated,the fire residues of immersion heater were investigated. The microscopic appearances of heating wire were observed by scanning electron microscope. The result indicated that the microscopic appearances in different conditions are not same. Therefore,the state of the immersion heater before fire can be distinguished directly and quickly by observing the microscopic appearances of heating wires. This provided a new technical method for the recognition of fire reason.