A novel soft contacting technology is proposed to reduce the risks of contacts that space manipulator is on-orbit service. The magnetorheological (MR) rotational damper is considered and utilized for cushioning and vibration reduction in the space manipulator. Based on the extended manipulator model, a linear dynamic formulation of free-floating space manipulator is built. Subsequently, the mechanical property of magnetorheological rotational damper is analyzed by using Bingham model. Then, the optimal control force can be obtained by using the linear quadratic optimal control theory. Finally, the optimal control force is served as the parameters to achieve the semi-active control of soft contacting by employing the clipped optimal control theory. The hard contacting and passive control technology are introduced to make comparison with the results of soft contacting. Some numerical simulations are made to demonstrate the validity and capability of the proposed soft contacting technology.