Abstract: Objective This study aims to evaluate the fracture toughness of coarse grain heat-affected zone (CGHAZ) of X80 pipeline steel in air and H₂S environment. It analyzes the effect of environment on the fracture toughness. Methods Three-point bending test is conducted to study the fracture toughness. The fracture morphology and crack propagation path of CTOD specimen are analyzed by SEM, and the influence mechanism of H₂S environment on fracture toughness of CGHAZ is explained. Results The results show that the fracture toughness of CGHAZ in H₂S is significantly reduced, and the average CTOD value is reduced by 72%, from 0.335 mm to 0.093 mm. The fracture morphology in the air is mainly dimple, while the fracture in the H₂S is mainly a mixed fracture feature of dimple and quasi-cleavage plane. The oxide of Ti-Mn-Fe is the initiation site of quasi cleavage plane. Conclusion Hydrogen-enhanced decohesion (HEDE) and hydrogen enhanced local plasticity (HELP) are the main mechanisms for the reduction of fracture toughness of CGHAZ in the H₂S environment.