Abstract: It is suggested that the same angle probe has different incident point in different materials. The test data show that the variations are very large and maybe seriously affect the accuracy of defect location. The comparative study on block CSK-IA of ultrasound path of the measured and the theory incident point reveals that the measured incident point has relation with velocity of material for test. Thereupon the equivalent sound path is of spindle shape is put forward, which varied with the refraction angle, and well explains the phenomenon.