Abstract: The increase in the tonnage of railway heavy-haul wagon led to a sharp increase in the crack rate of coupler knuckle. The existing research on the fatigue fracture life of the coupler knuckle still lacks test and theoretical support. Therefore，it is necessary to study the fatigue fracture life of coupler knuckle based on reliability. The fatigue bench test of the coupler knuckle is accomplished systematically based on the measured load spectrum of the line，and the fracture life is studied based on reliability theory. Then，from the perspective of fatigue damage distribution，the damage distribution of the dangerous parts of the coupler knuckle is analyzed in depth，and the mutual verification with the results of the bench test is achieved. The results show that the coupler knuckle mainly presents two failure forms of the inner wrist surface fracture and the lower traction flange fracture. The critical crack size at the root of the lower traction flange is 60 mm in length and 23 mm in depth，and the critical crack length at the inner wrist surface is 90 mm. With the reliability of 99.9%，the fracture life of coupler knuckle is 475 000 km. The fracture life in the case of inner wrist surface fracture is 1.04-1.75 times longer than that of the lower traction flange fracture. The difference in life is derived from accidental error but also conditional error. The maximum damage of the coupler knuckle is located at the root of the lower traction flange 42 mm from the side of the inner wrist surface. The damage at root of the upper traction flange is smaller than that of lower traction flange. The damage of the inner wrist surface is the least，but the area of fatigue cracks is the largest，which is easy to produce multiple small cracks and lead to fracture. The research work has important theoretical and engineering guidance value for the formulation of maintenance strategy and application security of heavy-haul wagon’s coupler knuckle.

Key words: heavy-haul wagon’s coupler knuckle, fatigue fracture, reliability, service life, critical crack size, significance test, fatigue damage