Abstract: The crack tip opening displacement test(CTOD)is one of important test projects of the submarinepipeline.Test samples are usually rectangular cross-sectional sample, as the sample is only part of the weldingjoint, and it can not fully reflect the fracture toughness of the welding joint.The full-thickness sample was preparedaccording to BS 7448: Part2: 1997requirements, and the CTOD test was done at a low temperature of-10℃, theCTOD value was compared with the result of rectangular cross-sectional sample, and a better fracture toughness wasgotten by the full-thickness sample, it was found that the CTOD was related to the stress state, chemicalcomposition and microstructure, the fracture toughness of full-thickness sample was obviously better than thesub-size sample, the full-thickness sample could better reflect the true fracture toughness of welding joint and guidefor engineering design.