For extensively existed high-aspect ratio structures in engineering, the bending-torsional-coupled vibration of the high-aspect ratio plate is studied for the first time. As the relative stiffness along the chordwise direction is far bigger than that of the spanwise, the chordwise section is simplified as a rigid body, while that in the spanwise direction is flexible. The Hamilton’s principle is applied to establish the governing equation of the bending-torsional-coupled vibration of the high-aspect ratio plate with simple supports at both ends. After projecting the governing equation into modal space via Galerkin method, the analytical results and numerical solutions of the bending-torsional-coupled vibration are obtained by the harmonic balance method and DQEM/DQM respectively. The convergence of the modal projection and the harmonic expansion are also verified. Finally, the influence of the external excitation on the forced vibration is studied. The results provide guidance for the design of high-aspect ratio structures in engineering.