Abstract In order to predict the fatigue life of couplers for heavy haul railway freight trains of the Datong-Qinhuangdao Railway in China, a model for calculating the crack initiation life of coupler knuckle is established based on the stress-strain relationship of coupler knuckle material and the theory of cumulative damage. Considering the local plastic effect of coupler structure, the stress dangerous part of coupler knuckle is defined by adopting the steady state method in the local stress-strain method. Based on the longitudinal coupler load spectrum of the 20,000-ton freight train under study and considering the randomness of coupler position, the mixed coupler load spectrum is obtained for the first time. The fatigue life of coupler knuckle of a particular heavy haul railway freight train is calculated. The life of coupler knuckle calculated by the method presented in this paper is consistent with the actual repair cycle of coupler, which can effectively minimize the impact of coupler crack fault on the train operation safety.