Small modular reactors represent one of the technological pathways for future advanced pressurized water reactors. As a passive safety measure, natural circulation plays an irreplaceable and significant role in small modular reactors. NuScale not only employs natural circulation in its passively designed safety features but also eliminates the main pumps, utilizing the driving force generated by natural circulation in the primary loop to circulate the coolant. To thoroughly understand the characteristics of natural circulation in NuScale's small reactor, a model was developed based on professional procedures for its primary and secondary loop systems, and dynamic changes in key parameters under normal operation and accident scenarios were calculated and analyzed. The results indicate that the NuScale reactor can achieve stable solely through natural circulation during full-power steady-state operation, effectively removing heat from the core; while in a shutdown state, two passive residual heat removal systems can continuously and effectively export decay heat into the reactor pool, demonstrating the system's efficiency and reliability. This further validates the feasibility of using natural circulation to remove core heat in small modular reactors and the safety of well-designed passive systems.