Abstract: Investigation the change rule of radius when ultrasonic probe lay in different location of cylinder during test angle welded joint with inserted take over, founded the calculation formula of the ellipse in which ultrosonic beam lay in. the relationship between display depth and actual depth as well as span according to the radius was investigated. Citing the angle welded joint with inserted take-over to calculate and draw curve draft of flaw location, using the draft we can locate the flaw, improve test efficiency and quality by using the draft.