Hybrid electric vehicle (HEV) technology represents a critical form of modern new energy automotive systems, with the hybrid powertrain serving as its core functional component. This study investigates vibration characteristics and shaft strength in heavy-duty HEV transmission systems based on multi-body dynamics theory. A compound torsional vibration model integrating the front transmission gear train, reduction gear set, and variable-speed gear set was constructed to resolve the system's natural frequency distribution and modal shapes. Critical parameters influencing low-order torsional vibrations were identified through contribution degree analysis, enabling optimization of critical resonance frequency bands and effectively avoiding system resonance. Dynamic load variations at critical nodes under engine-motor hybrid excitation were analyzed, with torsional vibration stresses in sensitivity-prone shaft segments calculated across full operating conditions under high-power inputs. Verification through comparative analysis between torque fluctuations at critical nodes and allowable stress thresholds confirmed that the strength of all key shaft segments meets operational requirements.