In this work, by using amorphous bimetallic sulfides of CoSnS_xPd as signal amplifier and magnetic material of Fe₃O₄ @PPyAu as the substrate, we fabricated an electrochemical immunoassay for Nterminal prohormone of brain natriuretic peptide (NTpro BNP). CoSnS_x was used as the signal amplifier of the immunosensor for the first time, which was synthesized by fast stoichiometric co-precipitation method. The binary sulfide was identified as an amorphous structure and performed apparent electrochemical behavior. The excellent performance of signal marker was induced by the synergistic effect between SnS₂ and cobalt, as well as the loaded Pd nanoparticles (NPs). Both CoSnS_x and Pd NPs presented excellent electrocatalytic activity toward H₂O₂ oxidation. Moreover, the magnetic nanocomposite of Fe₃O₄ @PPyAu was firmly immobilized on the magnetic glassy carbon electrode (MGCE) by the magnetic force. The magnetic substrate not only performed good electron conductivity, but also improved the stability of the fabricated electrochemical immunosensor. The developed immunosensor for the NTpro BNP detection exhibited a wide linear response (0.1 pg/mL to 50 ng/mL), and low detection limit of 31.5 fg/mL. In addition, the immunosensor held an excellent analysis capability and broad fascinating application for the detection of other biomarkers in medical diagnosis and treatment.