The abstract sound field is converted into visible image during this process which is helpful for the personnel to locate the position and size of the fault. This paper mainly focuses on the study of the spread of ultrasound in the material. It also explores the principles of ultrasonic propagation in the material according to the classical wave equation and uses the principles to interpret longitudinal transverse wave of ultrasonic. Firstly, the paper probes into the imaging method of ultrasonic flaw detection. Based on the time domain/transient analysis method, the analysis of harmonic response is made and the mathematical model of ultrasonic flaw detection is established by using the finite element software PZFlex. Then based on the established model, the simulation of ultrasonic in the material is studied. Finally, the position and size of the defects can be accurately determined by the specific ultrasonic testing experiments.