Improving the flexibility of coal-fired power units is the key to achieving a green and low-carbon power system. Molten salt energy storage technology, as a sensible heat energy storage technology, can provide frequency regulation, peak regulation, and industrial heating decoupling support for power generation units. Combining with the engineering application of a coal-fired power unit coupled with molten salt energy storage, the thermal performance of heat storage, heat release and energy cycling are systematically analyzed, and performance evaluation indicators are proposed. The results show that, the molten salt thermal storage system significantly enhances the flexible operation capability of the unit, achieves decoupling between industrial steam supply and deep commissioning peak of the unit, and improves the frequency regulation performance by 150%. The thermal efficiency of the thermal storage system is maintained at around 92%. This study provides a reference for the application of flexibility renovation projects for coal-fired power plants based on molten salt thermal storage technology.