Abstract: To the composite material, its thermal diffusivity is always changed with layer orientations and material temperatures, which makes it difficult to realize the automatic measurement of defect depth of composite material. To solve this problem, a method was provided, the thermal diffusivity of the detected material could be obtained from infrared thermal imaging series with non-linear fitting based on one-dimensional thermal conduction theoretical model, then combining the formula of depth calculatian, automotic measurement of defect depth of composite material was easy to be achieved.