Considering the even more complex system, varying operation boundary and working conditions i. e. load demand, coal quality and ambient temperature, the energy consumption of power unit changes in different parts with time. The concept of spatial-temporal energy-saving effect of thermal power units was proposed. The specific coal consumption theory was improved by considering the internal characteristics of equipment, the coupling relation among equipment and the dependant features between system and varying operation boundary. The energy-consumption benchmark state was determined under off-designed operation conditions. The case study on 1000MW supercritical power unit show that the energy-saving effect is related to the varying operation boundary and the energy consumption of each device is caused mainly by its own structure imperfection with the rest by the system topology.