We optimized a multistage electrochemical etching method for fabricating high-performance aluminum electrolytic capacitors using alternating current (AC) to maximize electrostatic capacitance while preserving the mechanical properties of high-purity aluminum foil. The etching process was systematically investigated under various AC frequencies, current densities, durations, solution compositions, and aluminum ion concentrations. Pretreatment and chemical modification with phosphoric acid (H3PO4) provided a highly uniform corrosion pit distribution, preventing localized over-dissolution. The optimal etching conditions were identified as a solution containing 350 mL/L HCl, 15 mL/L HNO3, 15 mL/L H3PO4, and 5 g/L Al3+, a temperature of 35 °C, a current density of 0.6 A/cm2, and a four-stage frequency-stepping profile of 30–30–25–20 Hz for a total etching time of 120 s. Under these conditions, the 50-μm-thick aluminum foil (99.99% purity) achieved a peak specific capacitance of 510.2 μF/cm2, indicating a >20% improvement over conventional single-stage methods. Microstructural analysis revealed a highly dense pit network with an average pit diameter of 180–240 nm, a nucleation density of 4.2 × 109 pits/cm2, an average surface roughness of 1.68 μm, and an exceptional surface enlargement factor of approximately 96.5. Lattice orientations of [220], [311], [200], and [422] in this order contributed to this surface expansion. Process reliability was confirmed by a low capacitance coefficient of variation (<1.2%), whereas the etched foil retained over 75% of its pristine tensile strength. Electrical evaluations demonstrated an exceptionally low equivalent series resistance of 0.12 Ω at 120 Hz, a superior frequency response (68% capacitance retention at 1 kHz), and a robust stability under accelerated life testing (85 °C at a rated voltage of 6.3 V for 1000 h) with less than 4.5% degradation. These results show that this multistage AC etching method directly supports the miniaturization, signal stability, and ruggedization demands of next-generation intelligent sensor technologies, particularly for localized power filtering in automotive modules and low-power wireless networks.