Abstract: Aiming at the fully automatic welded joints of X80M steel pipe under different preheating conditions, the difference of crack tip opening displacement is studied. The CTOD test is carried out on single notch three-point bending specimen, and CTOD value is calculated according to the formula recommended in GB/T21143-2014 and ISO15653-2018. The results show that: Compared with GB/T21143-2014, the results of CTOD calculated by formula recommended by ISO 15653-2018 are 35% higher on average. This difference is mainly due to the difference in the emphasis of the correction methods of the two standards, that is, the GB standard corrects the change of crack length, while the ISO standard adjusts for temperature effects. The CTOD value of the joint sample under the preheating condition of 80 ℃ is low on the whole. According to GB/T21143-2014, three samples are lower than the standard, the lowest is 0.16 mm, and according to ISO 15653-2018, one sample is lower than the standard, the lowest is 0.20 mm. Under the preheating condition of 120 ℃, the calculated CTOD values of all samples under the two standards meet the standard requirements, and the fracture toughness of welded joints is better. Under the same preheating conditions, the CTOD values of the fusion zone are higher than that of the weld zone, and the maximum value of CTOD values appearing in the fusion zone, which has good fracture toughness. However, the CTOD value of the fusion zone is affected by the inhomogeneity of the structure, resulting in great differences between different positions and uneven performance distribution.