Abstract: To address the fracture control problem of X80 unequal-diameter tee used in China-Russia Eastern Gas Pipeline under low temperature environment (－45 ℃), the finite element model for X80 unequal-diameter tee with defects was built taking into consideration the influence of wall thickness effect and structural stress concentration of the tee. The crack initiation toughness of X80 tee body and welds was calculated by applying the failure evaluation methods of Guide to methods for assessing the acceptability of flaws in metallic structures (BS 7910-2019) and Fitness-For-Service (API 579-1/ASME FFS-1-2016). Charpy impact toughness index of X80 tee at －45 ℃ was determined through the relation between fracture toughness and Charpy impact energy. The results show that the fracture toughness at the tee weld should be greater than 37.79 MPa·m^1/2, and the minimum value of each Charpy impact energy should not be less than 32.22 J. The stress intensity factor of the inner surface at the shoulder of the tee body is the largest, with the fracture toughness greater than 42.46 MPa·m^1/2, and the minimum value of each Charpy impact energy should not be less than 38.85 J. According to the requirements of domestic and foreign pipe fittings standards, the low temperature toughness index of the D 1 422×1 219 mm X80 unequal-diameter tee in China-Russia Eastern Gas Pipeline Project was determined.