Abstract: Corrosion of steel bars can cause structural damage or even failure, resulting in huge economic losses and casualties. The steel bars are located inside the concrete structure, and traditional monitoring methods are difficult to quickly, comprehensively, and accurately determine the corrosion of the steel bars. Ultrasonic guided wave monitoring technology, due to its long propagation distance and high information sensitivity, is suitable for monitoring small structural damage over long distances and identifying damage in multi-layer composite structures. However, when applied in the field of building structure detection, it is prone to environmental interference. In this paper, based on ultrasonic guided wave technology, a surface adhesive ultrasonic monitoring structure was designed for U-shaped steel bars, which improved the monitoring signal strength and a waterproof packaging was developed that could effectively avoid external environmental interference. After experimental testing, it was found that as the degree of steel corrosion changed, the velocity of ultrasonic guided waves changed. The amplitude and time consumption of the received signals were significantly changed, and the testing system can accurately determine the degree of steel bar corrosion.