Abstract: The prevailing theories for evaluating high-cycle fatigue damage in metallic notched components primarily focus on macroscopic phenomenological frameworks, with limited attention to descriptions at micro- and meso-scales. Elastic responses macroscopically characterize high-cycle fatigue, yet understanding how to account for the influence of notch effects while simultaneously considering the promotion of fatigue damage due to microplastic accumulation at the grain scale remains a significant challenge with practical engineering implications. Taking metallic notched plates as the research object, this study defines a critical domain at the notch root and establishes a critical domain energy conservation relationship by incorporating intrinsic microplastic dissipation and intrinsic damage dissipation. Based on mesoplasticity theories and scale transition criteria, the Lin-Taylor assumption is employed as a bridging mechanism, and a critical plane method is utilized to propose a volumetric averaging hypothesis, thereby achieving a macroscopic representation of microscale plastic dissipation. The theory of continuum damage mechanics describes the intrinsic damage dissipation behavior within the critical domain. With a single loading cycle treated as generalized time, a novel two-scale damage evolution model is developed. This model is implemented in ABAQUS through UMAT subroutine programming to calculate damage parameters under loading conditions for notched components. Finally, experimental data from six aerospace-grade metallic materials under various notch geometries and loading conditions, as well as comparisons with four classic models, are employed to validate and analyze the proposed model. The results demonstrate that the new model provides superior predictive performance. This research aims to further enrich the theoretical framework of multiscale analysis for metallic fatigue damage, endowing fatigue evaluation models with clearer engineering applicability and physical significance.