Mannich base corrosion inhibitors have good corrosion inhibition properties due to the presence of C=O, C–N, and other unsaturated factors, and they have excellent solubility properties. In the present study, a new type of Mannich base corrosion inhibitor (DFC) was synthesized with high-temperature resistance, corrosion resistance and excellent dissolution and dispersion properties. The static weight loss method evaluated the corrosion inhibition performance of the synthesized corrosion inhibitor. When the dosage of the corrosion inhibitor was 0.5%, the corrosion rate of Q235 steel was only 0.69 mg/(cm2·h) and the corrosion inhibition rate was 94.66% in 5% hydrochloric acid solution at 90 °C, which showed excellent corrosion inhibition performance. Using electrochemical methods, combined with thermodynamics, kinetics and other related theories, it was found that DFC is a cathodic type of corrosion inhibitor, and its adsorption behavior on the metal surface obeys the Langmuir adsorption isotherm. The type of adsorption is mainly physical adsorption.