<p>The Passive Residual Heat Removal System (PRHRS) of Fluoride-Salt-cooled high-Temperature Advanced Reactor (FuSTAR) has already been designed, and the transient safety analysis code FUSY was developed to perform the design benchmark accidents. However, the real transient thermal-hydraulic response of PRHRS still needs to be validated by the integral effect test facility. Considering the corrosion and high-temperature difficulties caused by the fluoride-salt of PRHRS, in this study, the heat transfer oil is used as the fluid of the test facility. Firstly, based on the Reynold transport theorem, the complete form of partial differential equations (PDEs) is derived without introducing additional assumptions. Subsequently, to understand the corresponding mechanism, the Hierarchical Two-Tiered Scaling (H2TS) method is adopted to scale the PDEs variables, which reflects the transient natural cycle process in accidents. According to the dimensionless parameters, the heat exchangers and pipelines are designed and arranged to satisfy the steady-state modeling. Finally, one dimension analysis was performed to verify the rationality and distortion of the modeling method. The results show that with the H2TS method, the steady-state effect of the natural cycle with the conjugate heat transfer process can be effectively modeled. In addition, the complete forms of governing equations of the natural cycle are independent of the type and regime of fluids, and there is no need to introduce any additional assumptions. This study provides a reference for the dynamic modeling of natural cycle loops and provides data for future experimental testing.</p>