In motor vehicle-related cases, criminals often modify the VIN (Vehicle Identification Number) of the vehicle frame to make the involved vehicles regain legal status. However, currently, there is a lack of technical means that can quickly and accurately identify whether the VIN code area has been tampered. This paper studies the use of ultrasonic methods to detect VIN code tampering. By employing appropriate probes and coupling agents, combined with wavelet transform noise reduction technology, reliable determination thresholds were explored and a non-destructive testing method was proposed. This method can quickly and accurately detect whether the VIN area of the vehicle frame has been tampered with by “replacement” through hidden welds, and can also precisely identify the size and location of weld defects. This portable non-destructive testing technology provides strong support for quickly investigating suspected stolen vehicles and effectively improves the efficiency of combating criminals.