Abstract: Analyzed here in paper is the regularity of the influence by the crack angle of the steel tube on the ultrasonic echo. Simulated steel tubes with different crack angles are designed so as to measure the echo amplitude and the defect length. As a result it is revealed that along with gradually increasing of the simulated crack defect angle and the axial angle of the steel tube，the defect echo amplitude first rapidly decays，and then continues，but slowly；after the defect angle increases up to 45°，the defect echo is close to the bottom wave，which makes it impossible to effectively measure the length of the defect. Therefore based on the fact that the crack angle obviously affects the UT result，the enterprise needs to adopts more other detecting methods to reduce the missed detection probability.