The substantial development of the power spot market, coupled with the enormous growth of clean energy sources including solar and wind, has significantly needs for operational flexibility of thermal generator. The implementation of flexibility upgrades in thermal generator can promote the generation profitability and market competitiveness as well as the flexibility of the power grid as a whole. As power demand changes, especially during peak and trough periods, the flexibility of thermal power plants is particularly important. Through the flexibility upgrades of thermal power, it can better adapt to the change of power demand and improve the overall efficiency of the power system. A method for evaluating the economic viability of flexibility upgrades of thermal generator is introduced. A chronological market simulation model is established and quantification method of the revenue generated by thermal generators participating in the power spot market is proposed. Furthermore, economic metrics are employed to calculate the investment cost recovery period. Case studies indicate that an optimal level of flexibility upgrades of thermal generators can significantly enhance their economic performance.