In order to investigate the excitation mechanism of thermo-elastic laser ultrasound and its application in defect detection, the temperature field and stress field in the material were calculated using the method of finite element analysis. The theoretical basis of the material irradiated by pulse laser was expounded, and the pulse laser was loaded on the surface of the workpiece in the form of heat flux density. Considering the convection and radiation heat transfer boundary conditions at the same time, the temperature field in the material was analyzed. Based on thermo-solid coupling, the temperature field was loaded into the stress field analysis process, and the directivity distribution of the body wave sound field based on the thermo-elastic mechanism was discussed. On the premise of simulating the interaction law of ultrasonic waves with surface defect and internal defect, the time history curve of the displacement of the node on the workpiece surface was extracted, and the obvious defect echo signal was obtained. With the purpose of verifying the results of finite element analysis, a laser thermo-elastic ultrasonic detection system was built using pulsed laser, ultrasonic transducer and oscilloscope for surface crack detection. The excitation of the ultrasonic wave is based on the thermo-elastic mechanism by controlling the energy of the pulsed laser. Through finite element analysis and experimental verification, the relationship that the reflection coefficient increases with the depth of the defect could be obtained. Furthermore, the inflection point of growth corresponds to the wavelength according to the maximum center frequency of the surface wave. A laser spot with a diameter of 1mm is used to detect cracks with a depth of less than 3mm. When the crack depth is greater than 2.2mm, the growth trend of the reflection coefficient becomes slower. The results of finite element analysis can provide reference and basis for the application of thermo-elastic laser ultrasound in defect detection.