：Crack Tip Opening Angle (CTOA) is considered and has been attempted to be used as a fracture parameter to describe the crack arrest toughness of ductile steel gas pipelines. However, its evolution and influencing factors during pipeline dynamic crack propagation are still unclear, and its use as a crack arrest toughness parameter similar to the Charpy impact toughness also lacks theoretical support. In view of the above problems, this paper reproduces the full-scale test with the help of the numerical simulation method for pipeline dynamic crack propagation. Comparing the numerical simulation results with the experimental ones, it is proved that the numerical model is reliable in studying the pipeline CTOA evolutions. On this basis, this paper systematically studies the evolution laws of CTOA in the process of dynamic crack propagation and arrest for natural gas pipelines, and clarifies the effects of the following factors on CTOA values, including the design factor, geometric dimensions, inertia backfill effect and material property of pipeline steel, et al. The results show that the critical CTOA (CTOA C) of a pipeline is independent of the design factor, the pressure decompression, and the large change in the crack velocity, and is also not sensitive to the pipe strength, pipe wall thickness, and the inertial effect of soil, but only related to the pipe diameter and pipe toughness. The above conclusions show that it is reasonable to use CTOA as the crack arrest toughness parameter and has laid a foundation for the subsequent establishment of the crack arrest control method for natural pipelines based on CTOA C.