Objective This paper aims to contribute to the field of ultrasonic non-destructive testing by establishing a model that can effectively detect different types of defects inside teeth. Methods The dG-FEM method is adopted to create an ultrasonic non-destructive testing model in COMSOL. By measuring various types of dental defects and measuring the same defects at different angles, the propagation law of ultrasonic waves in homogeneous media is explained. The results are presented in the form of ultrasound pulse emission and reception diagrams of the defects, as well as the comparison of detection signals and angle signals of different types of defects inside teeth. Results Different types of tooth defects can be detected within 16 microseconds, with the highest detection strength being observed in crack-type defects, at 22%. The detection signal for star-shaped defects is the weakest. Additionally, when the ultrasound source and the defect are aligned in the main direction, the detection strength is the highest, whereas the detection strength is the weakest when they are aligned in the same direction, with a value lower than 3%. Conclusions In conclusion, this study provides a comprehensive investigation of the potential of ultrasonic non-destructive testing in dentistry. With the establishment of an effective model and the demonstration of its potential, this study contributes to the development of new and more accurate diagnostic tools for dental practitioners.