In order to realize the infrared phase detection of debonding defects of concrete strengthened by bonded steel plate, the detection model is established according to the theory of heat transfer. The surface temperature is obtained by using the finite volume method, and the phase distribution is obtained by FFT. The relationship between phase difference and steel thickness, defect width, defect thickness as well as heat flux intensity is studied. The results show that the maximum value of phase difference decreases when the steel thickness increases with a certain defect size. For a given thickness of the steel plate and defect, the maximum value of phase difference increases with the increase of defect width. With a certain thickness of steel plate and defect width, when the defect height increases, the maximum value of detected surface phase difference increases to a certain extent, but the increase is small.Under certain steel thickness and defect size, the phase difference of the detected surface is almost unchanged when the heat flux increases. The results provide a theoretical basis for infrared phase detection of debonding defects of concrete strengthened by bonded steel plate.