Abstract: To address the problem of water leakage in a QT400-15 ductile iron cylinder head after approximately 80 h of operation, which was identified by inspection as an oxide-type (magnesia) spheroidized slag inclusion defect, the inclusion was simplified into an ellipsoidal model, and a finite element model of the cylinder head containing the inclusion was established for coupled temperature-displacement and XFEM-based crack propagation analysis. The results show that the thermal loading is the primary driving factor governing the evolution of the stress field, with a maximum overall temperature of 311.4 ℃. The maximum equivalent stress in the inclusion reaches 136.3 MPa, exceeding the yield strength of magnesia (105 MPa), indicating a significant risk of local failure. Cracks are concentrated on the side of the inclusion adjacent to the low-temperature passage. The failure of the inclusion induces microcracks in the matrix through interfacial stress concentration, which subsequently propagates to form a through-thickness crack causing water leakage. Based on a synthesis of literature corrections, the steady crack propagation rate at 600 ℃ is 19.5 μm/h, corresponding to a theoretical crack initiation time of approximately 200 h, and a comprehensive estimate of less than 128 h when the effect of the inclusion is considered. Owing to the limitations of the steady-state thermal-structural coupling analysis and the linear-elastic model, the results cannot be quantitatively correlated with the actual failure time of approximately 80 h; nevertheless, it provides a reasonable interpretation of the critical regions and failure mechanism, and can serve as a reference for the failure analysis of slag-inclusion-induced defects in cylinder heads.