The surface temperature is an important parameter capable of detecting and quantifying defects in laminated structures if subjected to a thermal load. In this paper, a temperature field reconstruction method is developed to detect the size of debonding defect in laminated structures. A Tikhonov regularization based conjugate gradient method (CGM) is applied to reconstruct the temperature field of a laminated structure for determination of the size of a debonding defect. Tikhonov regularization deals with the severe ill-posedness and the CGM is used for solving the minimization problem. Some simulation examples are carried out to validate the effectiveness and stability of the inverse analysis. Results show that for small defects, the defect detection results based on temperature field reconstruction have higher accuracy than that detected directly by available surface temperature measurement.