Abstract: P91 steel is widely utilized in critical structural components due to its outstanding performance in high-temperature environments. However, its cyclic softening behavior and the initiation mechanisms of fatigue cracks are not yet fully understood, which hinders the reliability design and lifespan prediction of the material. In this context, the present study proposes a crystal plasticity-based constitutive model grounded in physical mechanisms. The model systematically accounts for the evolution of back stress, martensitic lath coarsening, and dislocation slip/climb mechanisms, and is implemented using the finite element method. The simulation results effectively replicate the experimental observations of P91 steel under high-temperature cyclic loading, thereby validating the accuracy and applicability of the model. The findings indicate that the cyclic softening behavior of P91 steel at elevated temperatures is primarily driven by microstructural recovery processes, including martensitic lath coarsening and a reduction in dislocation density. Fatigue cracks are observed to preferentially initiate at grain boundaries or triple-junctions. The crack initiation life is predicted using two fatigue indicator parameters (FIPs): cumulative plastic slip and cumulative energy dissipation. The results demonstrate that both FIPs exhibit strong predictive capability, with all predicted data points falling within the two-fold error band. Under high strain amplitude conditions, the fatigue life prediction based on cumulative energy dissipation provides higher accuracy, underscoring its potential for application in high-temperature fatigue behavior research. This study provides a comprehensive scientific foundation for understanding the cyclic plasticity behavior and fatigue crack initiation mechanisms in P91 steel, and offers a robust methodological framework for the reliability design of high-temperature materials.