Abstract: Aiming at the detection and quantification of crack defects in pipeline inner wall opening, in this paper, studies have been made of the theoretical calculation and experimental results by using the ultrasonic testing end angle reflection theory, endpoint diffraction theory and phased array detection technology. The experimental results show that the height of micro crack on the inner wall of pipeline is linearly proportional with the height of reflected echo, and the end angle reflection method can effectively measure the height of cracks of being less than or equal to 4 mm in the inner wall of 20 mm thick pipeline. The endpoint diffraction method can effectively measure the height of crack which depth is not more than 2 mm in the inner wall of 20 mm thick pipeline. For pipes with different wall thickness, the minimum crack height measured by diffraction method is different. The above-mentioned findings are helpful for realizing the effective detection of pipeline inner wall crack.