The interactions between glycopolymer brushes and lectin are very important for the development of affinity membrane chromatography in protein separation. Here, we report the combination of surface-initiated atom transfer radical polymerization (SI-ATRP) and surface plasmon resonance (SPR) to investigate the relationship between the structure of glycopolymer brushes and the affinity adsorption of lectin. The glycopolymer brushes were fabricated from self-assembly of 11-mercapto-1-undecanol (MUD)/1-undecanethiol (UDT) mixture, immobilization of ATRP initiators, and then SI-ATRP of 2-lactobionamidoethyl methacrylate (LAMA). Brush thickness and grafting density were adjusted by controlling polymerization time and thiol ratio in MUD/UDT mixture, respectively. Sugar epitope density was also controlled through copolymerization of 2-hydroxylethyl methacrylate (HEMA) with LAMA. Ricinus communis agglutinin (RCA(120)), one kind of lectin that can bind galactose specifically, was chosen to study the effects of brush architectures on lectin adsorption. SPR results indicate not only the thickness but also the grafting density and the epitope density of glycopolymer brushes can achieve the best performance of sugar cluster effect in affinity adsorption of lectin. In addition, the mass transport effect is crucial in the adsorption process. We propose that it is important to keep the balance between the sugar cluster effect and the mass transport effect in the preparation of high-performance affinity membrane chromatography.