In this paper, the output voltage tracking control of DC-DC boost converters is studied. To meet the increasing safety requirements and enhance the control system robustness, overcurrent protection and disturbance rejection are investigated simultaneously. A simple and effective control scheme is proposed to achieve all these three goals, which includes two parts. In the first part, a disturbance observer is constructed to estimate multiple disturbances (including the input voltage fluctuations and the load variations). In the second part, a composite current-constrained controller is proposed by embedding the disturbance estimates and the nonlinear penalty term into the passivity-based control law. Under the novel control scheme, rapid tracking performance and safe overcurrent protection are obtained, even in the presence of multiple disturbances. Rigorous stability analysis is conducted to demonstrate the accomplishment of the output voltage tracking task and the strict guarantee of the current constraint. Simulations and experiments are performed to verify the effectiveness of the proposed control scheme.