The principle of a lock-in thermography for non-destructive tests was researched and the evaluation, defect depth measurement and its application to the detection of honeycomb structure materials and welding bearings were introduced.A Finite Difference Model(FDM) of 2D heat conduction was established and it then was used to compute the temperature variety process on the surface of samples under the condition of sine law modulated heat flux. The lock-in processing method was applied to extract the quasi-steady state temperature variety of the defect and non-defect regions and the phase differences between them was computed. Furthermore, a thermal-electronic equivalence modeling was established by analog to the fundamental laws of heat and electricity, and it was used to simulate the lock-in thermography. The relation between defect depth and phase difference of both incident and reflected thermal waves was obtained. Finally,non-destructive tests were carried out on a honeycomb structure sample with simulated defects and a real welding bearing by the lock-in thermography. Experimental results indicate that the phase deference between defect and non-defect regions obtained by the finite difference model and thermal-electrical equivalence modeling is close to the experimental result,and the bias error is less than 5% as well. The lock -in thermography can rapidly and exactly detect defect sizes and positions, and can be also used in the measurement for the curve surface of a structure.