Aspartic acid (ASP) and its derivatives are eco-friendly and cost-effective scale inhibitors but exhibit limited corrosion inhibition in acidic media. To enhance their performance against acid corrosion, a facile, purification-free one-pot aqueous reaction was developed to synthesize an L-ASPME/GA hybrid inhibitor from L-aspartic acid β-methyl ester (L-ASPME) and glutaraldehyde (GA). The resulting inhibitor solution was directly introduced into a 0.5 M H₂SO₄ pickling solution to achieve synergistic corrosion inhibition for Q235B steel. The corrosion inhibition performance was systematically evaluated using weight loss tests, electrochemical impedance spectroscopy (EIS), potentiodynamic polarization (PDP), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and contact angle measurements, with temperature effects also assessed. The results demonstrate that the L-ASPME/GA hybrid, particularly at molar ratios of 2:3–4:1, achieves 90.7%–96.1% inhibition efficiency, significantly outperforming L-ASPME or GA alone. Notably, the 2:3 L-ASPME/GA hybrid shows superior high-temperature acid corrosion resistance versus single components. This synergistic effect is attributed to a co-adsorption mechanism, forming a compactly oriented, thermally robust film driven by hydrogen-bonding networks, Fe²⁺ coordination, and electrostatic attraction. These findings offer a practical strategy to improve the acid corrosion resistance of ASP–like inhibitors.