The work aims to clarify the corrosion inhibition effect of a supramolecular complex (HPDA) (based on octade-cylamine (ODA) and hydroxypropyl-β-cyclodextrin (HP-β-CD)) on 20# mild steel in the condensate water, and illuminate the assembly mechanism on the pipeline steel. The inhibition effect of HPDA on 20# mild steel in the condensate water was evaluated through dynamic weight loss, electrochemical tests and surface morphology observation. In addition, X-ray photoelectron spectrometry (XPS) and static contact angles were applied to define the assembly mechanism of corrosion inhibition of HPDA on the pipeline steel. According to the results of dynamic weight loss, the corrosion process of 20# mild steel in the condensate water could be effectively retarded in the presence of HPDA. As the concentration of HPDA increased, the corrosion rate of 20# mild steel decreased along with the increased inhibition efficiency (η, %). The value of η could reach 94.11% after 72 h immersion in the condensate water at 40 °C with 100 mg/L HPDA. Besides, the value of η tended to be constant after 60 h immersion in the same concentration. From the results of potentiodynamic polarization, both the anodic and cathodic processes for the corrosion of 20# mild steel in the condensate water were inhibited in the presence of HPDA. The corrosion current was also reduced significantly after HPDA was added. HPDA could effective inhibit the corrosion and the corrosion inhibition effects could increase with the increase of concentration of added HPDA. HPDA was a kind of mixed corrosion inhibitor focusing on anodic process. From the electrochemical impedance spectroscopy, the corrosion of 20# mild steel in the condensate water was controlled by charge transfer process and was hardly altered by the addition of HPDA; but the polarization resistance could be hugely enhanced in the presence of HPDA. The analysis of surface morphology showed that no obvious corrosion was observed in the the sample of 20# mild steel after HPDA was added and the roughness closed to the newly polished sample. The surface of 20# mild steel in the presence of HPDA showed hydrophobic characteristics, but no valence linkage of host compound (Hydroxypropyl-beta-cyclodextrin) was detected, which indicated that only guest molecules (ODA) was assembled on the steel surface. The corrosion of 20# mild steel in the condensate water can be inhibited effectively by HPDA which can release guest molecules (ODA) and assemble the molecules to hydrophobic membrane on the mild steel surface to provided corrosion inhibition.