A ‘signal-off’ electrochemical immunosensor was designed for the ultra-sensitive detection of α-fetoprotein(AFP） based on the synergistic catalytic enhancement of metallic nano mimic enzyme to produce insoluble precipitation.The hollow gold-palladium-platinum nanoparticle bioconjugates(HAuNPs-PtPdNPs-Ab2） were immobilized on the electrode modified with electroactive substance nickel hexacyanoferrate nanoparticles through sandwich type immunoassay.In the presence of H2O2,HAuNPs and PtPdNPs as peroxidase-mimicking enzymes catalyzed 4-chloro-1-naphthol(4-CN） to generate insoluble and non-conductive precipitation benzo-4-chlorohexadienone(4-CD） at the electrode interface,thus effectively increasing the hindrance of electron transfer and leading to a significant decrease in electrochemical signal.The quantitative detection of AFP was achieved by the changes of electrochemical signal before and after the addition of AFP.Under the optimized reaction conditions,the experiment proved that the established immunosensor possessed a wider dynamic response from 0.1 pg/mL to 200 ng/mL in the detection of AFP with a low detection limit of 33 fg/mL.On the basis of the synergistic catalytic amplification,the proposed strategy could provide an effective method for multiple signal amplification to enormously improve the sensitivity of the biosensor.