A three degree of freedom dynamic model of the highline for underway replenishment system was investigated, which considered the influence of the incline angle of the highline and the periodical fluctuation of the tension of pendant vertex. The highline was truncated to one order model by Galerkin method. A three degree of freedom of ordinary differential equation system of the in plane oscillation of the highline system was gained by decoupling the inertial terms, and the system was numerically analyzed by Mathematica. The results show that the sway of solid cargo could induce the highline and solid cargo to oscillate in large range.