A three-degree-of-freedom nonlinear dynamic model of piecewise highline system during underway replenishment is investigated when the lumped mass swings in which the influences of piecewise characteristic of highline cable's static profile due to cargo deadweight, the incline angle and the horizontal motion of the sending end are considered. Considering 1:2 internal resonance and primary resonance, ordinary differential equations are gained by decoupling the inertial terms. Perturbation analysis and numerical analysis are carried out by adopting the multiple-scale method, which will be the theoretical basis of the further research of nonlinear dynamics.