Abstract: The fracture toughness test of low constraint specimens is of great significance for the safe operation of oil and gas pipelines. In this paper, the existing fracture toughness testing methods and the development process of low constraint specimens are reviewed, and the common fracture toughness characterization parameters such as crack tip opening displacement (CTOD) and J-integral are introduced. The key problems such as the stress intensity factor, the J-integral plasticity factor, the J-integral and CTOD conversion factor, the crack size measurement method, and the digital image correlation method, in the fracture toughness test, are analyzed, and the problems that need to be further studied are summarized, to provide a necessary basis for the development of fracture toughness test of low constraint specimens.