One of the solutions to reduce the sway oscillation motion of crane' cables is to use a passive damper. The application of this method is simple but limited in the insufficient flexibility to prevent adverse movements and to amplify the favorable ones. This paper proposes an on-off damping controller of semi-active damper to improve the passive damper. The aim of the on-off damping control is to amplify the radial motion of damper to increase the energy dissipated, then reduce the sway motion. The effectiveness of proposed controller are verified by numerical simulations of a 2D crane.