Abstract: By dividing the evaporating surface of supercritical once-through boiler into cold-water, density-change and overheat-steam three sections according to thermal properties of water and steam, a lumped-parameter model was built up for the evaporating surface, separator and superheater through mechanism analysis and model simplification, with which the heat storage coefficient of boiler was calculated under different loading conditions based on design data of boiler structure. Application results to a 1 000 MW supercritical unit show that the heat storage of working medium in boiler water wall varies more greatly along with the pressure at subcritical pressure than at supercritical pressure; the heat storage capacity of metal material is much higher than working medium; the superheater collects about 70% heat of the whole boiler; the heat storage coefficient of supercritical boiler reduces with rising pressure.