Abstract: Ultra-high aspect ratio unmanned aerial vehicles (UAVs) exhibit superior aerodynamic efficiency but are prone to significant geometric nonlinearities due to their lightweight and slender structures. Although three-dimensional finite element models can accurately capture these effects, their high computational cost limits practical application. This study presents an efficient dynamic modeling approach based on equivalent geometrically exact beams for ultra-high aspect ratio UAVs. Complex three-dimensional structures are reduced to a multibody system consisting of geometrically exact beam and rigid-body elements. Using the X-HALE UAV as a case study, fully coupled stiffness and mass matrices of the equivalent beams are derived through the symplectic transfer matrix method and kinetic energy equivalence. A flexible multibody model employing dual-quaternion-based beam elements is then developed to investigate static aeroelastic trim and transient dynamic behavior under cruise conditions. Numerical results demonstrate that the equivalent beam model closely reproduces the static and modal responses of the 3D FE model while significantly reducing computational cost and system degrees of freedom. The proposed method provides an accurate and efficient framework for dynamic simulation and design optimization of ultra-high aspect ratio UAVs.