Abstract: In order to investigate the influence of crack-pitting coupling on the time-varying mesh stiffness (TVMS) of a herringbone gear planetary transmission system (HGPTS), a calculation model of the TVMS of HGPTS under the influence of crack-pitting coupling was proposed, which was solved by potential energy method and integral method. The effects of crack-pitting coupling faults on the TVMS of the internal and external meshing pairs in the HGPTS are investigated. The results demonstrate that the TVMS of both the internal and external meshing pair of the planetary gear system decreases as the degree of crack-pitting coupling fault increases. When the crack-influenced zone is smaller than the pitting-influenced zone, both crack and pitting faults jointly affect the TVMS. However, when the crack-influenced zone exceeds the pitting-influenced zone, the change in the TVMS is primarily dominated by the crack fault, resulting in the largest decrease in value. Furthermore, the external mesh is more sensitive to the crack-pitting coupling faults compared to the internal mesh. These results can provide a theoretical basis for understanding the dynamic vibration characteristics of HGPTS.