•A cascade amplification strategy for electrochemical immunosensing was developed.•The method involves surface-initiated enzymatic polymerization, biobarcode probes assembly, and silver nanoparticles deposition.•This method can detect fetoprotein down to 0.046 pg/mL.•This new approach has good accuracy for the detection of AFP in clinical samples when compared with commercial tests.A cascade signal amplification strategy through combining surface-initiated enzymatic polymerization (SIEP) and the subsequent deposition of strepavidin functionalized silver nanoparticles (AgNPs) was proposed. The first step of constructing the electrochemical immunosensor involves covalently immobilizing capture antibody on a chitosan modified glass carbon electrode, which then catalyzes DNA addition of deoxynucleotides (dNTP) at the 3′-OH group by terminal deoxynucleotidyl transferase (TdT), leading to the formation of long single-stranded DNAs labeled with numerous biotins. Following the deposition of numerous strepavidin functionalized AgNPs on those long DNA chains, electrochemical stripping signal of silver was used to monitor the immunoreaction in KCl solution. Using α-fetoprotein as a model analyte, this amplification strategy could detect fetoprotein down to 0.046 pg/mL with a wide linear range from 0.1 pg/mL to 1.0 ng/mL. The achieved high sensitivity and good reproducibility suggest that this cascade signal amplification strategy has great potential for detecting biological samples and possibly clinical application.