In this study, the anti-swing control is investigated for a suspension cable system of a helicopter subject to external disturbance. First, a disturbance observer is designed to compensate for the effect of the external disturbance in finite time. Next, based on the proposed disturbance observer and the given desired trajectory, a unilateral boundary control law is proposed to eliminate the vibration by using Lyapunov's direct method. Under the designed control scheme, the ultimately boundedness of closed-loop is guaranteed. Moreover, the vibration range and trajectory tracking error will converge to a small neighbourhood of zero by selecting suitable parameters. Simulation results verify the rationality and validity of proposed control scheme.