It is extremely important to explore the synthesis methodology and application scope of functional polymer brush-based nanocomposites. In this work, an ultrasensitive sandwich-type electrochemical immunosensor based on a polymer brush/graphene oxide (GO) nanoprobe was designed for the quantitative detection of alpha fetoprotein (AFP). As the backbone of the polymer brush, polyacrylonitrile-g-GO (PAN-g-GO) was efficiently prepared via surface-initiated atom transfer radical polymerization (SI-ATRP). Benefiting from the large number of nitrile groups in PAN, the poly(vinyltetrazole-co-hydroxyethyl methacrylate)-g-GO (P(VT-co-HEMA)-g-GO) was synthesized through nitrile click chemistry. Furthermore, the immunosensor reached the design requirements of a high sensitivity as the P(VT-co-HEMA)-g-GO was connected to plenty of signal labels. Importantly, the designed immunosensor showed a wide range (2.5 × 10¹–5 × 10⁴ pg mL⁻¹) (R² = 0.993) and a low detection limit (LOD, ∼0.183 pg mL⁻¹) with a high selectivity, good reproducibility and excellent stability. Therefore, these polymer brush-based nanocomposites can provide potential applications for determining tumor markers (TMs) with robust signal amplification capability.