Abstract: The large-scale integration of high-penetration renewable energy into power grids has led to a significant decline in system inertia and a shortage of regulation resources, imposing ever-increasing pressure on the flexible regulation capability of thermal power units. This paper relies on a hundred-megawatt-scale hybrid energy storage demonstration project and establishes a multi-type hybrid energy storage and renewable energy coupled system consisting of grid-forming electrochemical energy storage, flywheel energy storage, and electric heating-molten salt thermal storage-power generation system. A flexibility quantitative assessment and coordinated control method for multi-type hybrid energy storage based on the transfer function equivalence principle is proposed, with comparative analysis against thermal power units. Firstly, a flexibility evaluation index system is constructed from four dimensions: regulation range, response rate, duration capability, and regulation accuracy. Secondly, based on the derivation of three sufficient conditions in the frequency domain equivalence, quantitative criteria including gain matching degree, frequency band coverage rate, phase matching degree, and comprehensive equivalence degree are established. Finally, a hierarchical collaborative control strategy is designed, and an electromechanical transient simulation platform is built for multi-scenario validation. The results demonstrate that the proposed assessment method can effectively quantify the substitution degree of hybrid energy storage coupled systems for thermal power flexibility, providing a quantifiable technical approach for flexibility assessment of large-scale renewable energy bases in desert and gobi areas.