This chapter focuses on the online analysis system for the thermal economy of NPPs, aiming to enhance operational efficiency through real-time monitoring and diagnostic technologies. The system framework includes modules for real-time data acquisition, thermodynamic performance calculation, online analysis, and result output. It integrates analysis of controllable/uncontrollable losses, equipment aging evaluation, and fault diagnosis to monitor key components such as steam turbines, condensers, and feedwater heaters. By comparing corrected efficiency with expected efficiency, operational deviations are identified, and optimization measures are proposed.