In order to find out the broken critical-position of coating interface, a theoretical analysis is made on the thermal field generated when the coating is scratched by infrared laser. The examined results of real-time temperature of coating being scratched by laser are analyzed. It is indicated that the change of surface temperature of the coating presents a two-phase process, which corresponds with the results of theoretical analysis. The turning point of temperature is just the critical position where the coating breaks off from substrate. The laser power is calculated to be about 31.92 W at the critical position. The result directly reflects the adhesion strength between coating and substrate.