The incremental relationship among cable force, displacement of cable end and cable length is derived on the basis of catenary equations. Also the displacement of the end-effector is related with the variance of cable force. Then dynamics of the system is established. A feedback controller is designed for the resultant force exerted on the end-effector using Lyapunov method. Combining with the dynamics of the system, a nonlinear PD controller is constructed where the adjustment of cable length is the control input. Numerical examples validate the methods presented.