The work aimed to synthesize a new diacetophenone imidazoline quaternary ammonium salt compound (labeled PPLB) and evaluate its corrosion inhibition performance for Q235 steel in 0.5 mol/L HCl solution by weight loss, potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) and scanning electron microscope (SEM). The inhibition mechanism of PPLB was also discussed. The diacetophenone imidazoline quaternary ammonium salt compound was prepared from organic amine, stearic acid, acetophenone and benzyl chloride by dehydration, cyclization, Mannich reaction and quaternization. The weight loss measurement results showed that with the increasing of PPLB concentration and medium temperature, the inhibition efficiency increased gradually, and when the inhibitor dosage was 4.8×10?4 mol/L, the inhibition efficiency reached to 98.9% at 353 K, proving that the PPLB was an excellent corrosion inhibitor for Q235 steel in HCl solution. Electrochemical studies indicated that the inhibitor simultaneously retarded both cathodic and anodic processes and acted as a mixed type inhibitor. The PPLB molecules absorbed on metal surface merely by geometric blocking mechanism and the corrosion reaction mechanism was not changed. Besides, the adsorption behavior of PPLB was in accordance with the Langmuir adsorption isotherm. After the addition of PPLB, the charge transfer resistance and film resistance values obviously enhanced, while the double layer capacitance and film capacitance significantly reduced, suggesting that the inhibitor molecules took effects by adsorbing at the metal/solution interface. The thermodynamic parameters deduced from experimental data revealed that the adsorption of inhibitor PPLB was a spontaneously chemical process. The inhibitor molecules may take effects by adsorbing on mild surface via lone pair electrons of nitrogen and oxygen atoms, which could lead to the increase of surface coverage of PPLB molecules on the Q235 steel surface. The inhibition performance was also evidenced by scanning electron microscope. Compared with the sample in 0.5 mol/L HCl solution, the surface of specimen in the presence of PPLB was much smoother. These results confirmed that the PPLB can effectively protect mild steels from corrosion in 0.5 mol/L HCl solution.