[Object] The boiler system has great potential of energy saving under deep peaking regulation and flexible operation conditions. [Method] In this paper, the boiler system of a supercritical secondary reheating unit is investigated. The mechanism of exergy losses of the boiler are introduced, and the causes and influencing factors of exergy losses in combustion and heat transfer were discussed and analyzed. The exergy in fuel, exhaust gas and heat transfer under different load conditions are studied based on exergy analysis method. [Result] When the load decreased from 100%THA to 20%THA, the exergy efficiency of boiler is reduced from 54.47% to 50.04%. Under different loads, the exergy loss ratio in combustion is the largest, and the exergy loss ratio of heat dissipation is the smallest. Exergy loss ratio of combustion is 57.3% at 100%THA load condition, which increases first and then decreases when load decreased. Exergy loss ratio of heat transfer is 41.2% at 100%THA load condition, which decreases first and then increases when load decreased. The sum of exergy loss ratios of combustion and heat transfer are between 98.1% and 98.5% under different loads. Among all the heat exchange surfaces in the boiler, the exergy efficiency of water-cooled wall is the lowest. Therefore, the exergy losses should be minimized from combustion and heat transfer processes. [Conclusion] The research have a positive significance for expanding the method of energy saving analysis of boiler system.