Phenacyl quinolinium bromide (PaQBr), an old-fashioned corrosion inhibitor in strong acid medium, was reinvestigated and a new indolizine derivative (DiPaQBr) was synthesized from PaQBr. DiPaQBr is classified as an indolizine quaternary ammonium salt. Based on high resolution mass spectrometry (HRMS) and nuclear magnetic resonance spectroscopy (NMR), the formula and chemical structure of DiPaQBr was accurately confirmed and characterized. The formation mechanism of DiPaQBr from the condensation of two molecules of PaQBr through 1,3-dipolar cycloaddition was proposed. The corrosion inhibition of PaQBr and DiPaQBr on N80 steel in 15% HCl was investigated by weight loss measurements, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). Polarization results indicate that the two compounds examined are mixed-type inhibitors. The adsorption of the studied inhibitors obeyed the Langmuir adsorption isotherm. The obtained results from gravimetric research as well as electrochemical studies indicated that compared with the inhibitive PaQBr, its indolizine derivative DiPaQBr exhibited a better corrosion prevention performance at a much lower concentration. The achieved results indicate that the derivative (the indolizine quaternary ammonium salt) of heterocyclic quaternary ammonium salts could inhibit the steel more effectively in acid medium.