A disturbance observer based robust control method is proposed to solve the trajectory tracking problem of the terminal grasp of a floating-based space robot with uncertain parameters and external disturbances. Based on the law of conservation of momentum， the second Lagrange equation is used to establish the dynamic equation of the system. Assuming that the external disturbance is an unknown time-varying variable， a disturbance observer is designed to estimate the total disturbance consisting of external disturbance and parameter uncertainty. Based on the estimated total disturbance， a disturbance compensation term is introduced to guarantee the control performance of the system. The stability of the proposed control law is proved. The control law can compensate the total disturbance caused by parameter uncertainties and external disturbances， thus improving the trajectory tracking performance of the system. Compared with the traditional robust control scheme， the proposed control scheme has various advantages including a simple controller structure， no need to measure the hinge acceleration and the position， moving speed and acceleration of the base of the manipulator， and less sensors needed in the system. Finally， the effectiveness of the above control scheme is verified by numerical simulation．