Objective To study the detection effect of different dental defects by using the finite element non-destructive testing method, and provide some theoretical support. Methods Based on continuous medium ultrasonic propagation method,the dG-FEM model was used in COMSOL software to simulate ultrasonic transmission and reflection patterns. The ultrasonic reflection intensity was monitored by changing dental defect types and ultrasonic detection angles inside the model. The time and intensity of detecting mutated ultrasonic waves were used to study the detection effect of dental defects. Results Dental defects could be detected within 16 μs by ultrasonic non-destructive method. For different types of dental defects, the best detection effect could be observed in rectangle-shaped defects. For different detection positions, when the ultrasound source and the defect were aligned in the main direction, the ultrasonic reflection intensity was the highest. Conclusions For defects of the tooth, the ultrasonic non-destructive testing method can be used for fast detection of defect types. The effect of defect detection can be increased by changing the detection angles, which provide theoretical references for its practical application.