This study aims to improve the signal-to-noise ratio (SNR) of chemiluminescence immunoassay (CLIA) by increasing the amount of protein immobilized on the surface of the magnetic bead (MB). Proteins are macromolecules with three-dimensional structures, and merely increasing the density of functional groups on the two-dimensional surface of the MB cannot significantly enhance protein immobilization. Therefore, we grafted spatially extended functional polymer to not only increase the density of functional groups on the MB surface but also expand their distribution in three-dimensional space, ultimately increasing protein immobilization. We synthesized poly(sodium methacrylate) (P-COONa) using atom transfer radical polymerization (ATRP) and examined its grafting density and behavior on the MB surface via nuclear magnetic resonance (NMR). Dynamic light scattering (DLS) results showed that grafting P-COONa increased the hydrodynamic radius of MBs, indicating its spatial extension. Next, we investigated the effect of P-COONa grafting on the immobilization of bovine serum albumin (BSA). Experimental results demonstrated that P-COONa grafting increased BSA immobilization from 19.9 to 42.92 mg/g. Finally, we evaluated the impact of P-COONa grafting on the SNR by detecting the D-dimer biomarker using CLIA. The results indicated that P-COONa grafting increased the low-value-to-background and high-value-to-background ratios by 4.21× and 17.17×, respectively. This study demonstrates that grafting spatially extended P-COONa increases protein immobilization on the MB surface, significantly improving the SNR of CLIA.