The condensate involving HS-, CO32−, Cl− and NH4+ usually causes severe corrosion of 20 steel in condensate stripping process of industrial coal gasification unit. The corrosion can easily lead to failure of pipe fittings and vessels, and result in economic losses and personal injury. To protect industrial coal gasification unit against corrosion, five inhibitors including imidazoline, imidazoline amide, mannich base quaternary ammonium salt, pyridine, and sodium molybdate are investigated with experiment of linear polarization curve, polarization curve, electrochemical impedance spectroscopy (EIS). The gas-liquid two phase experimental conditions mixed with suitable concentration of H2S, CO2 and NH4Cl are as close to the industrial alkalescent corrosion environment as possible. The experiment results indicate that two inhibitors of the mannich base quaternary ammonium salt and the pyridine have better effects on corrosion protection than others at the same injection concentration of 100ppm. A new compound inhibitor is developed based on the mannich base quaternary ammonium salt and the pyridine in terms of above results. The new compound inhibitor’s efficiency further reaches to 95.5%, suggesting that the new compound inhibition has better effect on corrosion protection than either of two single inhibitors above.