The work aims to reduce the corrosion of carbon steel in HCL solution and disclose the enhancing mechanism of corrosion inhibition of electron-withdrawing group (carboxyl) for 5-hydroxytryptamine (5-HT). Dynamic weight loss, potentiodynamic polarization curve and electrochemical impedance spectroscopy (EIS) measurements were performed to explore the corrosion inhibition effect of 5-HT or 5-hydroxytryptophane (5-HTP) for 20# carbon steel in 1mol/L HCl solution. By adsorption isotherm fitting, the nature of adsorption behavior of the two substances on carbon steel surface was clarified. Surface morphology was observed to verify the corrosion inhibition effect of 5-HT and 5-HTP for carbon steel. Based on density functional theory (DFT), the difference in deformation charge density between 5-HT and 5-HTP was calculated and compared. The corrosion rate of 20# carbon steel in 1mol/L HCl solution could be reduced effectively by 5-HT and 5-HTP and closely related to concentration and environment temperature. When 1 mmol/L 5-HT and 5-HTP were added at 298 K, the inhibition efficiency reached 92.19% and 95.76%, respectively. Potentiodynamic polarization revealed that after 5-HT and 5-HTP were added in corrosion medium, the corrosion current density reduced. In addition, as explained by EIS data, the interfacial charge transfer resistance was significantly enhanced in the presence of 5-HT and 5-HTP. The adsorption of 5-HT and 5-HTP on steel surface followed Langmuir isotherm model. Through comparison of dynamics and impedance parameters, 5-HTP had better corrosion inhibition effect than 5-HT for carbon steel in HCl solution. The outcomes of DFT calculations indicated that the protonation sites of 5-HT were electron deficient, while the electron density of protonated 5-HTP was uniformly distributed throughout the molecular skeleton. The electron-withdrawing effect of carboxyl group can promote the surplus charge to flow into protonation sites in the electron-rich region of 5-HTP molecule, so that the electron density is uniformly distributed in the molecular skeleton. Uniform electron density distribution is beneficial to the adsorption of 5-HTP on the surface of carbon steel in parallel configuration, covering the active sites to the maximum extent, and inhibiting corrosion efficiently.