Abstract: Numerical simulations were conducted on a domestic 1 000 MW ultra-supercritical double reheat boiler system based on a unit model newly built to study the performance of the boiler, and to calculate the exergy loss and exergy efficiency of the boiler system and its components at different loads according to exergy balance equations. Results show that with the variation of unit load, all the components maintain high exergy efficiency with small fluctuation, which could satisfy the requirements of deep peak regulation under low load conditions and ensure stable operation for the unit. The exergy loss caused by combustion and heat exchange accounts for 97% of the total, which could be lowered by reducing the heat exchange temperature difference between the working medium and the high temperature flue gas.