Abstract: In this paper, the true and false echo signals which appear in ultrasonic inspection of double-sided weld seams (including pipe butt girth weld, T-shape weld and angle weld) by welding from one-side are analyzed, and the causes and characteristics of the reflected echo signals are deduced. The method and technology of judging true and false echo signal are put forward, which has certain guiding significance for ultrasonic detection of weld seam with steel backing.