This study integrated an improved equivalent-input-disturbance (IEID) and a sliding-mode control (SMC) methods to ensure reference tracking and enhance disturbance-rejection performance for a Stewart platform. The internal principle ensures steady-state tracking of the system. A sliding-mode controller enhances the disturbance-rejection performance and tracking accuracy. The effects of the external disturbances are regarded as an overall disturbance. Then, a state observer and an IEID estimator estimate and compensate for the overall disturbance. The chattering phenomenon when implementing the SMC method is reduced because a small sliding-mode gain ensures the tracking precision in this control method. A stability condition of the closed-loop system is analyzed based on the Lyapunov stability theory. Gains of the IEID estimator and the state observer are designed by a linear matrix inequality that ensures the stability of the system. This method has been verified on an actual Stewart platform. Experimental results show that our method has better disturbance-rejection performance than an SMC method and an IEID method under external disturbances.