Abstract: Detection of thick steel plate by longitudinal wave is the main form of testing internal quality. We judge the quality of steel plate according to different standards and different level. For internal defects, some of them are allowed by standards, whereas some others are due to testing technique of longitudinal wave detection. The current shortcomings of qualitative and quantitative techniques can not fully meet the requirement of scientific researchers to understand details of defect in the steel plate, and an effort is therefore made to give the explanation of it.