<p>The long-shaft dual-rotor system exhibits heightened sensitivity to mass imbalance due to its high length-to-diameter ratio, frequently inducing unbalanced vibrations and structural fatigue. This study is the first to propose a dual-intermediate-bearing support strategy, comprising basic and improved configurations, to address this issue. A dynamic model is established using Timoshenko beam finite element theory, incorporating rigid disks and bearing elements. The inherent dynamic characteristics under both traditional and the proposed dual-intermediate-bearing support schemes are derived theoretically. Under rotor imbalance excitation, the system response is solved using the Newmark-HHT method. A full-node analysis is conducted to examine the time-domain responses, orbital trajectories, and frequency spectra. The vibration peak nodes of the inner and outer shafts are identified for analyzing amplitude-frequency and three-dimensional spectral characteristics. Further analysis focuses on these peak nodes within the improved configuration to compare the dynamic responses throughout the acceleration, steady-state, and deceleration phases under the three support schemes, revealing distinct dynamic behaviors. A sensitivity analysis is performed on key parameters, including bearing stiffness and shaft length. Model validity is verified through comparison with established literature and cross-validation with results from the harmonic balance method. The results demonstrate that the dual-intermediate bearing support significantly improves the natural frequencies and critical speeds while effectively suppressing vibration amplitudes, with the improved configuration exhibiting superior performance. Different support configurations show distinct parameter sensitivities. This study provides an important theoretical foundation for the structural optimization, parameter matching, and vibration suppression of long-shaft dual-rotor systems.</p>