Abstract: This paper explores the effect of defect gradient on ultrasonic guided-wave′s propagation in flat strap.Ultrasonic testing results show that the defect gradient has great influence on the strength of received signal.The smaller the gradient is,the weaker the received signal strength is,and the signal can be as weak as it cannot be distinguished from the noise.This is mainly because the reflected waves in different depth own different phases,which leads to wave superposition and weakens the intensity of reflected signal.