In this study, a high-performance anti-fouling coating based on poly adenine (polyA) as well as a highly specific cluster of differentiation 20 (CD20) epitope mimetic peptide (CN14) were employed to synergistically construct a facile biosensor for the rapid and sensitive determination of rituximab in lymphoma patients' plasma. The well-designed and optimized polyA coating displayed excellent stability, hydrophilicity, thanks to its intrinsic affinity with gold surface and thoroughly exposed hydrophilic phosphate groups. Moreover, the proposed strategy avoids the necessity to modify binding groups (e.g. thiol), making it more facile, repeatable and efficient. When dealing with complex clinical plasma samples, the polyA coating demonstrated better anti-fouling performance and lower background signal in comparison with mercaptan and bovine serum albumin coatings. The dissociation constant (~60 nM) between CN14 and rituximab was measured by microscale thermophoresis and their binding mechanism was further explained using computer simulation. The constructed GE/CN14/polyA biosensor displayed satisfactory performance with detection limit of 35.26 ng/mL. Finally, the proposed biosensor was successfully applied for rapidly determining rituximab in lymphoma patients' plasma, and exhibited comparable accuracy to the commercial ELISA, but has advantages including a shorter detection time, wider detection range and lower cost. It's worth noting that the anti-fouling polyA coating can be tailored according to the surface property of sensing interface and can be easily expanded to other gold electrode related biosensors.