It will raise their feedwater temperature and then improve their operating economy to add NO.0 high-pressure heater (HPH) in thermal power units. In this paper, a steam-water distribution model for a thermal system coupled with NO.0 HPH is established based on the mass and energy conservation laws; and a variable-condition calculation method for its heat economics is also proposed. Furthermore, a derivative characterization approach is developed to represent the heat economics impact of NO.0 HPH extracted steam flowrate, which realize the quantitative evaluation before and after coupling with NO.0 HPH. Taking a 600MW unit for example to carry out calculations and analysis, it indicates that the index calculation error of the method proposed in this paper is less than 0.15%, which has high precision and can be used for thermal economic evaluation of large thermal power units coupled with No. 0 HPH; furthermore, after coupling NO.0 HPH, the feedwater temperature can be lifted by 19.1-32.3℃, the cycle heat efficiency raised by 0.18%-0.2%, and the heat consumption rate decreased by 29.3-34.6 kJ/(kW·h). Moreover, the sacrificial internal work will be even smaller, and the heat consumption decrease be even larger if the turbine load was smaller.