In order to ensure safe operation of heavy haul train and maintenance cost control of coupler,sub-models containing elliptical surface crack are established on fatigue crack zones of the coupler body and knuckle. Finite element method and interaction integral method are combined to analyze the stress intensity factor of the crack’s front edge in different parts under three opening modes,and identified the main propagation form; The stress intensity factor on different crack propagation stages in the traction platform of coupler body and traction surface of knuckle are analyzed and ensure the propagation path. The results show that the main propagation is I mode in traction platform and tail hole of coupler body,traction face and traction platform of knuckle,the crack propagation in lock chamber of coupler body is mainly the composite mode of II type and III type. With the length increase of crack,the stress intensity factor at the surface tip in traction platform zone and the knuckle traction surface is greater than the value in the crack depth tip,and when the crack extend to a certain size,the length of the crack along the surface is faster than the depth direction,and the actual simulation of crack propagation path is similar to the actual growth path to verify the accuracy of numerical simulation.