This work aims at developing a thermal wave imaging method for testing the insulation layer of superconducting busbar in thermonuclear experimental reactor. In the paper, a film bag filled with hot water is used to heat the sample surface. The approximate theoretical model of transient heat conduction under an instantaneous cylindrical surface source in cylindrical structure is given. Through simulation and analysis of the theoretical model, it is found that differential processing of the thermal cooling signals can improve the signal contrast between defects and non-defects areas. Compared with the flash pulse excitation method, the contact excitation method can detect deep defects.