Abstract: The high proportion of renewable energy has affected the generation of thermal units and brought strong volatility.Thermal units are often at deep regulation state and are susceptible to disturbances, resulting in poor boiler heat storage levels and insufficient frequency response capabilities, posing a threat to system frequency stability. This paper focuses on the impact of dynamic changes in heat storage of thermal power units. Based on DC power flow and classical boiler models, a transient analysis model for the frequency of interconnected systems considering heat storage dynamics of boilers is constructed. By means of model equivalence aggregation, equivalent transformation, solution loop and approximate substitution, the order of the model has been reduced. Furthermore, based on the characteristics of the model, modal decomposition is carried out to derive a simple closed-form solution to the two-machine equivalent model. Based on the closed-form solution, the impact of the parameters on the heat storage model of the thermal power boiler on the transient frequency process of the interconnected system is analyzed. The calculation example shows that the established model and the proposed analytical method can accurately take into account the impact of heat storage of boiler on interconnected systems in frequency transient calculations, and the results can provide references for enhancing the frequency support capability of thermal power units.