Composite inorganic-organic acid magnesium corrosion inhibitor was designed and synthesized with stearic acid, palmitic acid, oleic acid, naphthenic acid, and active magnesium oxide as the main raw materials. The application performance and mechanism were tested. The results show that the composite magnesium carbonate-magnesium naphthenate corrosion inhibitor has a good corrosion inhibition effect. When the dosage is 1000 μg/g, the sulfur trioxide content in flue gas is reduced by 70.0%, and the corrosion rate of the steel sheet is reduced by 21.6%. Further analysis shows that the synthesized product is a stable colloidal system. This colloidal system was formed by nanoscale magnesium carbonate particles encapsulated by organic acid magnesium micelles. This effectively solves the problem of poor dispersion of magnesium carbonate particles in fuel and facilitates the synergistic effect of inorganic-organic magnesium salts. As the fuel burns, vanadium pentoxide, which can catalyze the conversion of sulfur dioxide to sulfur trioxide, combines with magnesium oxide produced by the decomposition of corrosion inhibitors. The concentration of vanadium pentoxide decreased, so the content of sulfur trioxide in flue gas decreased, ultimately inhibiting low-temperature corrosion. At the same time, the presence of magnesium oxide makes the type of vanadate generated by combustion change from low melting point to a high melting point, which alleviates high-temperature corrosion.