Abstract: The basic theory and elements of the blooming melt of TSICO steel billet on-line measurement system is presented. The on-line non-contact measurement system adopts the infrared imaging technology. The program detects the steel billet's edge by computing the grads and searching the maximum grads, then subdivides the edge according to the grads. A new image processing technology is particularly presented ,which is applied to this non-contact measurement system. With the mathematic analysis and test result, the new image processing technology is demonstrated to be practical and robust.