This chapter systematically explores thermal balance analysis methods for NPP thermal systems, including traditional approaches (efficiency method, heat balance method, cycle function method, equivalent enthalpy drop method) and matrix analysis. It details the calculation steps of the fixed-power method, covering thermal equilibrium models for core components like reactors, steam generators, feedwater regenerative systems, and condensers. Iterative computational processes are demonstrated through case studies. The analysis highlights that condensers account for over 65% of total thermal losses in NPPs, emphasizing the need to enhance regenerative and reheat cycles. Additionally, it compares methods like the equivalent enthalpy drop and cycle function approaches, providing theoretical insights for optimizing thermal efficiency.