A method automatically separates out the defect image from a thermogram sequence based on the physical characteristics of the defect of a conductive material in electromagnetic field. Defect area radiates more heat than other area, when it is mapped to the histogram of the image to be separated, the defect area is located in the top-end of histogram, and the proportion of defect area is smaller to the background or other area. The method equally divides the histogram of the image to be separated into multi groups, and calculates the first derivative ki of total pixel number Si of group i, finds the maximum absolute value of ki, i.e. |ki|max, where i is expressed as imax; if imax is the last group, Wn is regarded as threshold T, otherwise, Wimax+1 is selected as threshold T. The pixels that less than threshold T are removed to obtain the defect image in ECPT.