Abstract: Because of corrosion and pressure and other reasons, the bottom surface-breaking crack often occurs to the big weld bead in pressure equipment. In this paper, the height of bottom surface-opening crack is measured by ultrasonic phased array technique. Base on the echo of the two pointed end of bottom surface-opening crack, the absolute arrival time technique is proposed to measure its height. The block with different height of the artificial cracks is made. The height of artificial cracks in two blocks is measured to verify the precision in test simulation and experimental measurement. The result shows that the 3 mm height of bottom break crack can be measure quantitatively by the absolute arrival time technique (AATT) from one side and can achieve the same accuracy as the conventional TOFD. The influence of beam deflection angle on AATT is also discussed in this paper. The result shows that based on the diffraction wave of crack tip, the AATT has higher measuring accuracy.