Abstract: To improve defect positioning accuracy, this paper discusses various reasons for the advanced artificial reflector echo on the compared test block and reaches the following conclusion: nonstandard of the test block and the degree of lower half-angle of spread is smaller than the probe refraction angle are two primary reasons. So in the practical sensitivity adjustment, defect sizing should be on the basis of dB value of the artificial reflector highest echo and also highest echo position on upper and lower surface should be the reference condition on defect depth position judgment. It will be helpful to improve the defect positioning accuracy and avoid defect misjudgment.