Abstract <p>Helicopter high-power gear transmission system usually operates under high-speed and high-load conditions, while its motion complexity and strong nonlinearity bring challenges to its dynamic design. Unreasonable dynamic parameters often cause to large dynamic loads and poor nonlinear dynamic behaviors, further endanger the stability, maneuverability, and service life of the system. To match the engineering practice and enhance the accuracy of dynamic modelling and fatigue life/load prediction, to start with, a 41 DOFs global bending-torsion-lateral coupled nonlinear dynamic model of the helicopter entire and full-path transmission system was formulated with consideration to multiple nonlinear factors and gears' lateral vibration DOFs in three spatial directions. Then, a series of nonlinear numerical analysis methods were applied to analyze the influence patterns of system's key parameters on its nonlinear dynamical behaviors, including multi-periodic hopping, Hopf bifurcation, period-1 motion, chaos, and quasi-periodic motion. More importantly, through innovative application of global bifurcation analysis to fatigue life/load prediction frameworks, the effects of the motion instability and hopping of nonlinear dynamic states on the system's dynamic load coefficients and service life which fluctuate greatly in the chaotic region and the vicinity of bifurcation points were revealed. Finally, the critical motion-state transition threshold of the system with parameter changes were obtained, which provides significant theoretical and technical support for the dynamic design and control of helicopter high-power transmission systems.</p> Graphical Abstract <p></p>