The work aims to synthesize three imidazoline corrosion inhibitors with different carbon chain length for corrosion protection of N80 carbon steel and explore the effect mechanism on the corrosion protection performance in carbon dioxide miscible oil displacement environment. The imidazoline corrosion inhibitors with C7, C11 and C17 carbon chains were synthesized by amidation and cyclization reaction with octanoic acid (OA), lauric acid (LA), stearic acid (SA) and diethylenetriamine (DETA), respectively. Fourier transform infrared spectroscopy (FTIR), quantum chemical calculation, weight loss method, electrochemical measurements including electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization curve (PDP), Raman spectroscopy and surface observation technologies were employed to study the anti-corrosion mechanism of the synthesized three corrosion inhibitors. FT-IR results proved that imidazoline corrosion inhibitors with different carbon chains were successfully prepared. Quantum chemical calculations showed that the synthesized C17 imidazoline corrosion inhibitor had the best electron donating ability and hydrophobic ability. The results of weight loss indicated that all the synthesized inhibitors had good inhibition effect on N80 steel in CO2 miscible oil displacement corrosion environment. The inhibition effect of inhibitors increased as the inhibitor concentration and carbon chain length increased, and reached over 90% for imidazoline inhibitor with 17 carbon chain at 10 mg/L. The EIS results indicated that the capacitive arc radius increased with the increase of imidazoline concentration and carbon chain length. The polarization curve results suggested that the imidazoline corrosion inhibitor behaved as an anodic corrosion inhibitor in the test environment, and its self-corrosion current density decreased sharply with the increase of imidazoline concentration and carbon chain length. It could be seen that the same trend of inhibition efficiency was obtained by the method of weight loss and electrochemical test. Raman spectroscopy results indicated that corrosion inhibitors with C7, C11 and C17 carbon chain length were adsorbed on the N80 surface after immersion in test solution for 48 h due to the C=N and C—N character absorption peaks. The electron pushing effect of alkyl could enhance the adsorption effect of inhibitor onto steel surface. The SEM results showed that the surface of N80 added with C17 imidazoline was smooth and flat, and grinding marks could be clearly observed. This result proved that the imidazoline with C17 carbon chain length had the best corrosion protection effect compared to those with C7 and C11 carbon chain length. The adsorption film formed by inhibitor with longer carbon chain had better hydrophobic effect, which led to the better inhibition effect of synthesized imidazoline inhibitor. The synthesized imidazoline corrosion inhibitor with C17 carbon chain has excellent corrosion protection effect for N80 carbon steel. With the increase of carbon chain length, the electron donating ability of the imidazoline is enhanced, which makes the imidazoline corrosion inhibitor easier to be adsorbed on the surface of N80 steel. At the same time, the longer the carbon chain of the imidazoline corrosion inhibitor is, the better the hydrophobic effect of the adsorption film formed on the surface of the carbon steel is, so imidazoline inhibitor with C17 carbon chain has the best protection for N80 steel.