Abstract: The control rod assemblies perform the indispensable and critical function of power regulation and emergency shutdown in nuclear reactors, forming a fundamental safety barrier. The reliability of their free fall behavior, often referred to as the “drop time” directly impacts overall reactor safety and operational integrity. A typical control rod assembly consists of a central spider coupling and 24 individual neutron-absorbing control rods. The free fall process is defined as the assembly starting from an initial zero velocity, descending under the primary influence of gravity within a confined guide tube, and culminating in the contact between the lower end surface of the connector rod and the limit stop. However, accurately predicting this dynamic behavior presents significant theoretical and practical challenges. The operating environment within a reactor core is exceptionally complex, characterized by extreme conditions such as high temperature, high pressure, intense radiation fields, and pervasive flow-induced vibrations. These factors profoundly affect the assembly’s motion. Furthermore, the descent is governed by strongly coupled multi-physics phenomena. These include repeated collisions and friction between the control rod assembly and the surrounding guide channels, complex vibrational effects arising from the system’s inherent structural flexibility, and the significant, transient coupling effects with the surrounding fluid’s resistance. The intricate interplay of these factors dramatically increases the difficulty of creating accurate theoretical models using conventional analytical approaches. To address these limitations, a novel, high-fidelity and fluid-rigid-flexible coupled multibody dynamics simulation method was developed and implemented. This research focused specifically on a typical control rod assembly, establishing a comprehensive simulation model for its free fall behavior that explicitly integrates both transient fluid load effects and nonlinear mechanical contact interactions. For the fluid dynamics part, an advanced computational fluid dynamics (CFD) software was utilized to simulate the complex surrounding flow field, pressure distribution, and associated hydrodynamic forces acting on the moving assembly. This involved modeling the narrow annular gaps and the resulting viscous effects. Simultaneously, the detailed mechanical system, including the spider arms, individual rods, and their interactions with the guide channels, was modeled using a multibody dynamics (MBD) software. This model accurately captured contact forces, friction, and structural flexibility. A critical aspect of this approach was the implementation of a robust co-simulation strategy. This technique enabled real-time data exchange between the CFD and MBD solvers at each time step, thereby faithfully capturing the two-way coupled interactions between the fluid and the structure. The results demonstrate that our proposed coupled method provides a substantially more accurate and physically realistic simulation of the control rod assembly’s dynamic response under these challenging, complex boundary conditions. It successfully captures transient events like bouncing and jamming that are difficult to predict with decoupled models. Consequently, it achieves a more precise and reliable prediction of the total drop time, a key safety parameter. This work offers valuable, high-fidelity theoretical support for enhanced nuclear reactor safety analysis and provides critical, data-driven insights for the structural optimization of control rod drive mechanisms and guide tube designs. The developed methodology also establishes a reliable reference framework for peers investigating similar complex fluid-structure interaction problems in other tightly constrained engineering environments.