<p>With the advancement of aerospace technology, satellite payload units are evolving toward higher device density and longer mission durations, placing urgent demands on thermal management. Consequently, efficient and accurate thermal design has become crucial in satellite engineering. The thermal resistance network model (TRNM), as a medium-fidelity yet computationally efficient approach, has demonstrated advantages in system-level analysis, but has not yet been applied to payload units. This paper focuses on applying the TRNM to the thermal design of task modules within satellite payload units. It details the modeling process, including defining the minimum modeling scale, discretizing structures into nodes and thermal resistances, adapting boundary conditions for plug-in units, and exploring its application in intelligent optimization. The accuracy of the TRNM was verified through both finite element modeling and experimental testing, offering a fast response and reliable methodology for thermal design of modern microsatellite payloads. For future work, the TRNM can be extended to support more complex thermal designs, such as heat dissipation plates integrated with phase change materials or high thermal conductivity composites, as well as transient thermal analysis scenarios.</p>