Abstract: In order to ensure the quality and safety of steel structure, in the premise of not affecting the weld quality, fast and accurately to the weld scan, K2.5 or K3 probe is applied to perform single-sided unilateral testing with direct wave method, perform wave mode analysis and qualitative as per groove type of welds and defect wave position in welds, and internal defect of welds can be detected effectively, thus achieving accurate and quick detection to welds of H steel structure.