This paper addresses an on-line parameter adaptation for a momentum accumulation control of a space robot in the post-grasping of a tumbling target whose dynamic parameters are unknown a priori. The model inaccuracies in the target lead to an unexpected tumbling motion after grasping a target. It is desired to transfer the entire angular momentum to the reaction wheels as quickly as possible while stabilization trajectory of the robot-arm is tracked to avoid self-collision in the chaser-robot. Firstly, we derive a momentum control method from the angular momentum equation to accumulate the entire angular momentum into the reaction wheels. The parameter inaccuracies degrade the control performance. Then, an online adaptation law by using coupling force and momentum is proposed. A numerical simulation is carried out to verify the operational performance of the proposed method in the presence of model uncertainty.