In order to avoid the failure of bolted joints used on ultrasuper critical steam turbine inner casing, their mechanical behavior should be strictly assessed, especially under the inservice conditions with steam temperature and pressure fluctuations. In this study, a 3D bolted inner casing is modeled using the commercial finite element software ABAQUS. The Norton-Bailey constitutive model and strain-life-based Manson-Coffin equation are applied to study the mechanical behavior of the bolted inner casing under in-service conditions. The results demonstrate that the inner casing could exert significant influence on the bolts by conduction, structure restriction and thermal expansion difference of the materials. The temperature and von Mises stress of the bolts experience continuous fluctuations during the steady state operation phase due to the constant variation in the steam temperature and pressure, and the alternative stresses give rise to fatigue damage of the material. The steam temperature and pressure fluctuations decline the fatigue lifetime of these high-Temperature bolts in long-Term operation, which should be taken into account in the design.