Abstract: Based on the low-temperature fracture toughness test results of Q690 high-strength steel, the failure assessment diagram(FAD) was used to evaluate the low-temperature fracture failure of Q690 high-strength steel base metal and weld specimens with the thickness of 32 mm and 50 mm from two aspects of failure assessment point and loading path. The results show that at the same temperature, high-strength steel with a thickness of 50 mm is more likely to reach the safety assessment critical point than 32 mm, and the safety assessment point of the weld is closer to the safety assessment curve than the base material. As the temperature decreases, the safety assessment point of Q690 high-strength steel gradually approaches the safety assessment curve. When it reaches around-60 ℃, the change is rapid, indicating that the brittleness of the material rapidly increases and the safety decreases below this temperature. In terms of loading path, as the temperature decreases, the type of material failure gradually shifts from plastic failure to elastic-plastic failure and brittle fracture failure. The external loading required for failure is relatively smaller, and the loading required for 50 mm specimen failure is greater than that for 32 mm specimen, which is consistent with the material ductile brittle transition mode caused by low temperature. The failure mode of the 32 mm Q690 high-strength steel base material changes from ductile failure to mixed failure mode below-40 ℃, and enters brittle failure mode after reaching-80 ℃, while the 50 mm base material enters brittle failure mode near-60 ℃. In contrast, the welds of the same thickness are more likely to enter mixed failure mode and brittle failure mode than the base metal.