: Aluminum alloy 2219 (AA2219) friction stir welding (FSW) joints for rocket propellant tanks are subjected to cryogenic tensile loading, and crack initiation during deformation plays an important role in the evaluation and prediction of their service performance. To investigate the crack initiation behavior of AA2219 FSW joints at 77 K, a crystal plasticity finite element model incorporating hard second-phase particles and interfaces was developed in this study. Three different initiation mechanisms associated with the aluminum matrix, particles, and interfaces were considered. The developed model was validated using the cryogenic tensile stress–strain response. Based on this model, the crack initiation behavior of AA2219 FSW joints during cryogenic plastic deformation was analyzed. The results show that crack initiation in the aluminum matrix is mainly affected by the localization of accumulated plastic shear strain near grain boundaries, especially in regions with large crystallographic misorientation. Particle cracking is primarily caused by maximum principal stress concentration within the particles, and elongated particles whose major axes are nearly parallel to the tensile direction exhibit a higher cracking tendency. Interfacial debonding is closely related to non-uniform stiffness degradation. Regions with high damage preferentially develop along relatively straight grain boundaries, whereas triple junctions and particles hinder their further propagation. Accurate prediction of plastic damage behavior in aluminum alloys therefore requires the coupled consideration of matrix plastic slip, brittle particle fracture, and interfacial degradation.