Abstract: In order to solve the problem of testing interior flaw height of welded joint by using single probe, tip diffraction method was introduced to measure flaw height hiding in the welded joint, and the result was compared with TOFD. Comparing the numerical value difference measured by software of TOTD and calculated by tip diffraction method, the conclusion of practicability and accuracy was obtained in testing interior flaw height of welded joint by tip diffraction method, at the same time, the tilt angle of flaw orientation and detection surface was calculated by the method in this paper.