This paper is devoted to the voltage regulation of a DC-DC buck converter within current constraints. Existing methods often fail to maintain adequate dynamic performance while adhering to this constraint. At the same time, the control performance is significantly degraded due to disturbances such as parameter uncertainties, input voltage fluctuations, and load impedance variations. In order to solve these problems, a control design scheme based on a gain function is proposed by skillfully combining finite time disturbance observation and finite time control technology, which ensure that the inductor current is kept within the safe range to prevent hardware damage. The whole closed-loop system is guaranteed to be finite-time stable. The synthesized results from simulation and experimental data show that the proposed control strategy is superior to the conventional methods regarding current constraint, dynamic performance and disturbance rejection.