Abstract: High pressure, rich gas transmission pipeline requires the development of measuring method of Crack Tip Opening Angle (CTOA). Using quasi-static tear propagation and specimen of long crack propagation length, the crack propagation experiment of X70 pipeline steel was conducted. The whole progress of propagating crack was recorded by camera, the CTOA were directly tracked from the photographs, the behavior of fracture mechanics of crack propagation was analysised. The test results show that the thinner the ligament thickness is, the more easily the steady-state crack propagation process of thin ligament is obtained, and the CTOA of steady-state crack propagation process increases as the ligament thickness increases. CTOA of steady-state crack propagation process of 4mm and 8mm ligament thicknesses are 7.6° and 11.3°, respectively, and the ratio of crack propagation length and the ligament thickness are 4―25 and 4―10, respectively.