<p>Time-varying mesh stiffness (TVMS) is one of the important excitations in a herringbone planetary gear system (HPGS). This study aims to investigate the effect of crack-pitting coupling faults on the TVMS of the HPGS. An analytical model (AM) is developed using the energy method to consider different crack scales and the degree of pitting influence on each gear pair of the HPGS. The integral method is employed to solve the AM. The effects of crack-pitting coupling faults on the TVMS of the HPGS are separately examined for the sun, planet, and ring gear. The calculated results of the TVMS are analyzed and compared for perfect, single fault, and crack-pitting coupling fault conditions. Furthermore, the TVMS of the HPGS under crack-pitting coupling faults at four different degrees is determined using the finite element method (FEM), and the results are compared with the AM. The findings reveal that coupling a 10% crack with seven pits reduces the TVMS by 4% compared to the perfect gear. Coupling a 30% crack with 14 pits reduces the TVMS by 32.5%. When a 60% crack is coupled with seven pits, the change in TVMS is predominantly influenced by the crack fault, similar to the calculated value of TVMS under a single-crack fault. The maximum error compared to the FEM results is 2.63%, confirming the accuracy of the results. Additionally, the TVMS of the external meshing pair is found to be the most sensitive to coupling faults. The results of the study can provide a theoretical basis for the dynamic stability and fault diagnosis of the HPGS.</p>