This work presents a novel approach to improving the load flexibility of coal-fired power plant by integrating high temperature thermal energy storage (HTTES) through additional thermodynamic cycle. It aims to investigate the feasibility of introducing an HTTES-aided additional cycle into the steam-water cycle of conventional coal-fired power plant and to test how a high round-trip efficiency can be achieved with the HTTES integration. The thermal system and operation mode of an HTTES-aided coal-fired power plant were described. The performance evaluation model was built and its performance was discussed based on a 600MW subcritical coal-fired power plant model. The results show that the proposed approach is feasible to improve the load flexibility of the conventional coal-fired power plant. An additional net power can be obtained while the original unit works at its rated condition. The initial steam pressure of the additional cycle is the determining factor for the round-trip efficiency and a round-trip efficiency of 44.18% can be obtained. This work is expected to provide a detailed reference on the use of HTTES system to improve the load flexibility of coal-fired power plants and further to relieve the serious problem of the curtailment for fluctuating renewable power generation in China.