It will raise their feedwater temperature and then improve their operating economy to add No. 0 HPH (high-pressure heater) in thermal power units. A steam-water distribution model for a thermal system coupled with No. 0 HPH was established based on the mass and energy conservation laws. And a variable-condition calculation method for its heat economics was also proposed. Furthermore, a derivative characterization approach was 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 600 MW 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.