Abstract: After welding the steel plate is generally fused together with the weld seam. On ultrasonic testing of the weld defect, echoes from the weld root were normally mixed together with the structural echoes from the back plate position, which should bring certain difficulty for the correct judgment of the defect echoes for on-site testing. By summing up the test experience and test verification, an effective echo identification method was deduced, which could effectively distinguish between the above-mentioned echoes.