Abstract: For the applications of Lock-in thermal wave imaging technology, the paper is focused on its theory, simulation and experiment. The relationship between the coating thickness and phase was analyzed through the theoretical curve. It was concluded that coating thickness had a linear proportional relationship with the phase in a certain thickness range, and calculation results show that Lock-in thermal wave imaging can effectively measure the coating thickness. The phase locked thermal imaging detection system was developed for coating thickness measurement, and the system was used for coated specimens. The experimental results show that the PLL frequency is 0.17Hz for the detection of 20-150 μm coating thickness with good repeatability and the relative standard error is less than 2.5%. This paper has guiding significance for the application of infrared thermography in the field of the coating measurement technology.