Based on thermal elastic excitation principle,two different sets of detecting systems are used to detect defects of metal plate with coating,and laser ultrasonic test methods are studied. Firstly,the feasibility and validity of this technology are verified by the experiments,and the influence of the coating on defects is discussed. Secondly,the defect waves under the effect of the coating are theoretically analyzed through experimental data,and then the parameters of defect waves are calculated in real time,and the shape and dimension parameters of the defects are obtained under the effect of the 50 μm coating. The results show that laser ultrasonic test technology can qualitatively and quantitatively detect the defects of coated metal plate. Thus the technology can also be applied to on-site measurement in the engineering field,and has good development prospects.