One-step purification of biomolecules from complex feed is challenging with commercial membrane adsorbers since it is difficult to screen and regulate the ligands on the membrane with traditional techniques for removal of various impurities. By readily grafting cationic polyelectrolyte ligands on a polydopamine (PDA)-coated membrane, a membrane adsorber with suitable ligand (750 kDa polyethylenimine, PEI) was selected to achieve one-step purification of arantitrypsin (AAT) from pretreated human plasma fraction W. It was found that ligand structure and molecular weight showed a significant effect on the properties and separation performance of membrane adsorbers. The attached polyelectrolytes increased membrane hydrophilicity but caused an enhancement of filtration resistance, and quaternization of the ligands (PEI and polyallylamine) further improved the hydrophilicity but decreased the charge quantity. By further screening elution salt concentration, flow rate, buffer pH, loading volume as well as molecular weight of PEI ligands, a high activity recovery of 96.6% and a high purity of 94.6% of AAT were obtained by one-step membrane chromatography, which was even better than those by multiple-step method. This could be owing to the special multilayer structure of PEI chains, which could suppress the impurity protein binding on the adsorption sites by exclusion effect. The present work not only provides new insights into the purification of biomolecules from complex streams with polyelectrolytecoupled membrane adsorbers, but also offers a new approach to design the ligands on membrane adsorber for specific application.