The problem of position tracking for output-constrained piezoelectric actuator (PEA) systems with disturbances is investigated in this paper. A robust output-constrained controller is proposed via a disturbance observer (DOB), so as to improve the position tracking performance in the presence of disturbances. The DOB is designed to estimate the lumped disturbance including the non-linearities, external disturbance and parameter uncertainties of the PEA. By virtue of the barrier Lyapunov function (BLF) and the feed-forward disturbance compensation, a novel composite controller is designed to compensate for the disturbances while guaranteeing pre-specified constraint of the position tracking error. Simulation tests verify that the proposed control scheme improves the position tracking performance and effectively satisfies the output constraint in the presence of disturbances.