Dynamic coupling is an inherent behavior of the space robot system, which seriously affects the motion accuracy of the system. This paper focuses on how to guarantee the base attitude stability of semi-floating space robots, from a control perspective, namely, the base attitude decoupling control. The quaternion representation of the system dynamics is derived to avoid the representation singularity of the Euler angles. In consideration of the complicated system dynamics and the limited computing ability of the on-board computer, an efficient decoupling controller is designed based on the time-delay estimation (TDE) and the super-twisting control (STC). Herein, for the proposed TDE-based STC (TDE-STC) scheme, TDE is used to decouple and linearize the nonlinear dynamics, and STC is a second-order sliding mode control (SMC) algorithm which can compensate for the TDE error and drive the state variables to converge to the equilibrium point robustly in finite time. The model-free decoupling principle of TDE is clearly illustrated by comparison with the classical computed-torque control (CTC). The global asymptotical stability analysis of the closed-loop system is proven using the Lyapunov theory and the linear matrix inequality (LMI) theorem. Finally, several comparative simulation studies on a three-dimensional (3-D) space robot system with consideration of joint friction, disturbances, and model uncertainties are conducted to verify the effectiveness of the proposed TDE-STC scheme. The corresponding results show that the TDE-STC scheme can achieve a high-accuracy attitude decoupling performance with both the model-free merits and the chatter-free merits.