Abstract: A research was performed on high square-wave pulse emission technology. Together with high-gain high signal-noise ratio amplifier, high-speed low-power consumption digital processing system and dedicated low-frequency probes, large-diameter graphite rod was inspected by ultrasonic through-transmission method. Comparision test was carried on using common ultrasonic flaw detector with low-frequency ordinary probe. The result shows that: using this new technology, the surplus sensitivity is two times of normal ultrasound method while using this new technology; and the 340mm large diameter graphite rod inspection problems are successful resolved.