To address fatigue crack propagation and life assessment of coupler knuckles under heavy-haul conditions, a crack growth life prediction method based on stress intensity factor range (ΔK) evolution is proposed. First, statistical analysis of in-service locomotive coupler data is performed to identify crack-prone regions and spatial distribution. A three-dimensional finite element model of the coupler knuckle is established to obtain stress field under loading conditions, and key analysis regions are determined with crack initiation locations. A three-dimensional surface crack model is introduced to evaluate stress intensity factor distribution along the crack front. Crack growth behavior is described using Paris law, and an incremental integration-based fatigue life prediction model is developed. Results show that cracks are mainly concentrated at the pin hole edge and traction lug transition region, consistent with high-stress regions from finite element analysis. The stress intensity factor range ΔK shows non-uniform distribution along the crack front, with peak concentration within about 1 mm beneath the surface. With increasing load eccentricity, the ΔK distribution is reconfigured, leading to higher crack growth rate and reduced fatigue life. The proposed method enables fatigue life prediction from crack initiation to failure. Compared with traditional nominal stress or empirical damage models, the proposed method describes three-dimensional non-uniform evolution of ΔK at the crack front and establishes relation between crack driving force and remaining life under load eccentricity conditions.