The mechanism of wind-induced oscillation of hangers in a long-span suspension bridge was investigated qualitatively on the basis of field observations, and the possibility of wake galloping, or vortex-induced resonance, or wind-rain-induced oscillation was excluded by reasoning. Numerical simulation and theoretical analysis reveal that this is a kind of resonance induced by the buffeting of main cables. Stochastic wind fields excite the stochastic responses of the main cables, which are abundant in a wide range of modal components, and the resonance occurs when the natural frequency of a hanger is adequately close to one of the main cable’s natural frequencies that possess enough energy. Hence, it is not the hangers themselves but the main cables that are responsible for the energy absorbing from the turbulence. Compared with the modal mass of a main cable, the mass of one hanger is too small to pose substantial effects on the oscillation of the main cable, and therefore, a steady supply of energy from the main cable to the hangers can be formed. Furthermore, TMD mounted on the main cables can suppress the wind-induced vibration of cables at the natural frequency of hangers, which results in the significant reduction of the vibration of the hangers.