Abstract: Under the new power system, the large-scale grid-connected power generation of distributed energy such as wind and solar energy leads to a sharp increase in the demand for flexible adjustment resources. The rapid development of flywheel energy storage technology provides an effective solution to frequency security and stability of power grid. Aiming at the application of flywheel energy storage system coupled to thermal power units in the field of frequency modulation, this paper studies the energy storage technology of power class high-power maglev flywheel from the research idea of "key technology R & D - system scheme design - coupled frequency modulation control", and designs the electrical connection and flywheel integration scheme of fire-storage coupled frequency modulation. Finally, combined with the engineering practice, a control scheme of coupled large-scale flywheel energy storage group cooperative frequency modulation for thermal power units is proposed. At present, this technology has been successfully applied to two 600 MW subcritical thermal power units in Lingwu Power Plant, which has improved key power quality indicators such as primary frequency regulation and automatic generation control(AGC). The qualified rate of primary frequency regulation of the combined system has increased by 21.26%, and the comprehensive index of AGC regulation performance has increased by 16.4%. The total annual income of the project can reach 35.148 million yuan, which has achieved good economic and social benefits. It is a successful practice of flywheel energy storage system in the power field.