This paper studies a linear system with unmatched disturbances. A new constrained disturbance observer-based tracking control scheme is constructed, where a robust command governor is employed to provide the modified reference command enforcing the state and input constraints. The primary ingredient of the robust command governor is the safe set, which reduces the conservatism by tightening the constraints using disturbance estimation error bounds. Simulation results of the attitude tracking control for a missile demonstrate that the proposed method satisfies the constraints despite the disturbances and achieves asymptotical command tracking.