Abstract To achieve carbon peak and neutrality targets, improve energy efficiency and reduce power generation costs, it is particularly important to study the thermal economy of thermal generating units under variable operating conditions. Taking the 330 MW subcritical thermal generator unit as the research object, based on the deep mining of SIS operation data, combined with the testing of key parameters, the thermal characteristics of the boiler, pipeline and turbine are deeply analyzed, and the key factors of heat loss of the unit are studied. The heat input, loss and output are calculated and counted in detail, the heat balance data relationship of the unit thermal system is established, and the heat utilization and loss of the unit are clarified. The main parameters of the thermal system were calculated and analyzed to study the main reasons affecting coal consumption and corresponding optimization measures. Through the research and analysis of thermal characteristics, heat utilization, and economic indicators of coal consumption, the targeted measures of energy saving and consumption reduction can provide a scientific basis for the daily scientific management and operation of thermal generating units and equipment transformation.