In this work, a new electrochemical immunosensor was developed for prion protein assay based on hybridization chain reaction (HCR) with hemin/G-quadruplex DNAzyme for signal amplification. In this amplification system, the hemin/G-quadruplex DNAzyme simultaneously mimicked the biocatalytic functions for H₂O₂ reduction and L-cysteine oxidation. In the presence of L-cysteine, the hemin/G-quadruplex catalyzed the oxidation of L-cysteine to L-cystine. At the same time, H₂O₂ was produced under the oxygen condition. Then, the hemin/G-quadruplex could quickly catalyze the reduction of H₂O₂, mimicking the catalytic performance of horseradish peroxidase (HRP). Under the optimal conditions, the immunosensor showed a wide linear response range from 0.5 pg/mL to 100 ng/mL with the low detection limit of 0.38 pg/mL (3σ). By changing the specific antibody, this strategy could be easily extended to detect the infectious isoform of prion (PrP^Sc) and other proteins. Based on its good analytical performance, the developed method shows great potential applications in diagnosis of prion diseases at presymptomatic stage and bioanalysis.