Abstract: Nuclear thermal propulsion technology is widely recognized as a transformative propulsion technology for future deep-space exploration, offering high specific impulse, favorable thrust-to-weight ratio, and long-duration reliability. However, the transient flow control of the working fluid and its dynamic coupling with reactor power during the startup phase remain underexplored, posing critical challenges to the engineering implementation of NTR systems. The startup process involves complex interactions among supercritical hydrogen flow, heat transfer, core power evolution, turbine dynamics, and nozzle response, processes characterized by strong coupling effects. To address these challenges, the system-level coupling characteristics during the NTR startup process were systematically investigated in this study. A steady-state inverse solution model of the entire NTR system was developed. This model integrated key components, including the reactor core, turbopump, bypass valve, preheater, and nozzle, enabling a coupled performance solution across different startup stages. The evolution of critical parameters, such as loop flow distribution, bypass valve opening, core temperature field, turbine output, and nozzle impulse, was thereby quantified. Subsequently, the key inlet conditions (temperature, pressure, and mass flow rate) derived from the steady-state model were employed as inputs to drive the point reactor kinetics equations, enabling a full transient simulation of the reactor startup process. This sequential coupling of steady-state component solutions with transient core dynamics effectively resolved the interaction between system-level flow distribution and reactor power evolution, overcoming the limitations of conventional decoupled models. The results demonstrate that the reactor core can achieve a smooth and controllable semi-automatic startup under specific operating conditions, with no abrupt power transitions during the ascent phase. During the low-power stage, propellant preheating proves essential to mitigate excessive density reactivity feedback: Insufficient preheating results in overly dense hydrogen entering the core, introducing positive reactivity that can trigger rapid power surges. Concurrently, bypass-driven loading is indispensable to address turbine power deficiency under low-pressure and low-inlet-temperature conditions, where the mainstream flow alone cannot provide sufficient driving work for the turbopump. In the high-power stage, increasing the turbine bypass valve opening is necessary to divert a portion of the high-temperature, high-pressure gas, thereby mitigating thermal and mechanical stresses on turbine components and ensuring operational stability. In conclusion, this work reveals the underlying coupling mechanisms between flow control and reactor power during NTR startup, providing a theoretical foundation for cooling loop design, startup strategy formulation, and transient condition definition. Furthermore, the proposed steady-state inverse solution and transient simulation framework are adaptable to other nuclear propulsion systems, offering broader technical support for deep-space exploration initiatives.