The invention provides an infrared thermal imaging technology based measurement method, and in particular provides an infrared thermal imaging technology based iron shape detecting method. The methodis characterized in that infrared light transmitted from the surface layer of an impact sample is collected based on the theory that metal materials have thermal radiation difference; different thermal images can be obtained based on different transmitting rates of \ infrared light transmitted from a crystal area and a ductile area; boundary points of the crystal area can be found from the thermalimages, and then a fracture of the impact sample can be measured. According to the method, the thermal imaging treatment technology is carried out; the manual fracture measuring and analyzing operation is replaced by the high-accuracy infrared measuring system based infrared thermal image measuring method, thus the shortage of high influence of the temperature and environment on image collectingof simple CCD cameras can be overcome, and the fracture shape can be measured in metal charpy impact test in the field of metal material test; and meanwhile, the calculated area of the crystal area can be used for measuring the fiber section rate and the side expansion rate of the fracture, drawing a ductile-brittle transformation curve, solving Tk, and measuring the outline dimension of the sample as well as the gap size.