The construction method of the heat release signal of a supercritical boiler is proposed, which can accurately measure the calorific value of coal in the furnace. The thermodynamic property standard, IAPWSIF97 is adopted to partition the supercritical boiler water cooled wall, which solves the difficult problem of judging the working fluid state within the water cooled wall under supercritical condition; and the boiler heat storage coefficient is calculated in detail. According to the structure parameters and actual operation data of a certain 1000MW unit,the heat release signal of the structure was verified. The results show that the heat release signal can rapidly response to the unit load change, at the same time accurately reflect the changes of the calorific value of coal in the furnace, which lays a foundation for the design of the direct energy balance coordinated control system of supercritical units.