Subspan oscillation on bundled conductors is one of the most important failures of overhead transmission lines, which is due to the leeward conductor lying in the wake of the windward conductor. In this paper, considering the aerodynamic nonlinearities, a 2-DOF subspan oscillation dynamic model of the leeward conductor lying in the wake of the windward conductor is given firstly. And then, the incremental harmonic balance method (IHB) is used to derive high-order limit cycle oscillation of subspan oscillation system, and the first to the third harmonic responses of the limit cycle oscillation are obtained. Results show that subspan oscillation only exists in a certain range of wind velocity. Moreover, the higher order of the harmonic the weaker impact it is, and the first harmonic response agree with numerical results of Runge-Kutta well. Lastly, the influence of span and original position of the leeward conductor on subspan oscillations has been analyzed, which can provide the technical support for suppression or prevention subspan oscillation.