With the integration of high shares of renewable energy into power systems and the deepening of market-oriented electricity reform, fixed cost recovery for conventional generating units and system capacity adequacy have become critical issues. Given this background, a staged capacity adequacy assurance mechanism adapted to the deep transition of the power supply structure is proposed, covering three evolutionary stages: fixed capacity compensation in the pilot stage, category-specific auction-based capacity compensation in the transition stage, and a unified capacity market in the mature stage. First, a fixed capacity accounting method based on expected cross-market revenues is developed to support fixed cost recovery for essential flexible generation resources in the initial stage. Then, a category-specific auction-based capacity compensation model is established according to the regulation characteristics of heterogeneous resources, guiding different types of flexible resources to bid independently. Finally, a centralized clearing mechanism for the unified capacity market is designed to coordinate long-term capacity investment decisions. It is demonstrated through case studies that the proposed staged mechanism supports effective fixed cost recovery for gen-eration companies, guides rational investment in flexible resources such as new-type energy storage, and ensures long-term system capacity adequacy and flexible regulation capability.