Abstract: An automatic ultrasonic testing system based on virtual instrument technology was introduced including its structure, inspection theory, hardware and software design and the application in welded steel pipe production process. It combined traditional ultrasonic inspection technology with some advanced ones such as industrial control computer, virtual instrument, digital signal process and so on. With the well-programmed defect detecting operation and flaw identification programme, the system could evaluate correctly and select strictly for complex flaw echo, and the dynamic review of defect echo waveform function overcomed the difficulty of flaw classification in ultrasonic automatic inspection, and achieve faulse call rate of less than 3%, and the undetected erroe rate as low as to zero.