To address the challenges posed by time-varying system inertia and the insufficient adaptability of conventional thermal-storage frequency regulation strategies under high renewable penetration, this paper proposes a coordinated thermal-storage frequency control strategy based on online inertia estimation and adaptive deadband optimization. The strategy employs a hierarchical coordination mechanism: under small disturbances, energy storage systems—acting as fast, distributed flexible resources—are prioritized for response through a reduced deadband setting, thereby avoiding frequent cycling and wear of thermal units; under large disturbances, the equivalent system inertia is identified via inversion of the frequency response, enabling adaptive adjustment of the storage’s virtual inertia and droop coefficient to dynamically compensate system damping. Furthermore, a full-lifecycle cost model incorporating cycle-life degradation is established to quantify the economic benefits of the proposed strategy. Simulation results demonstrate that the approach effectively mitigates system oscillations while significantly reducing overall frequency regulation costs, offering a technically and economically viable solution for distributed energy storage participation in grid ancillary services and frequency stability management in low-inertia power systems.