The design trend of the insulating layer of soft magnetic composite materials (SMCs) is to consider both their soft magnetic properties and resistivity. In this study, using Fe/Ni_0.5Zn_0.5Fe₂O₄ composite system as an example, the influence of MnO₂ oxidant on the soft magnetic properties and resistivity of the samples is studied, and the redox mechanism resulting from spark plasma sintering (SPS) at SMCs interface which improves the soft magnetic properties and resistivity is revealed. The Fe@Ni_0.5Zn_0.5Fe₂O₄(MnO₂) core-shell composite powders with different amount of MnO₂ (0, 0.1wt%, 0.3wt%, 0.5wt%, 1.0wt%) are prepared by ball milling, the Fe/Ni_0.5Zn_0.5Fe₂O₄(MnO₂) bulk SMCs are prepared by SPS. Their structures are characterized by SEM and XRD, and their electromagnetic properties are tested by precise resistance tester and vibrating sample magnetometer. It is found that the resistivity of Fe/Ni_0.5Zn_0.5Fe₂O₄(MnO₂) bulk SMCs with 0.5wt% MnO₂ is 33.7% higher than that without MnO₂, the saturation magnetic induction strength is 6.9% higher. The results show that the SPS sintering enhances the rapid REDOX reaction at the SMCs interface, and the addition of MnO₂ oxidants changes the ions concentration of ferrite at the SMCs interface, shows multiple effects of simultaneously improving the soft magnetic properties and resistivity of SMCs, because it reduces the B-bit electron transition frequency, increases the effective Bohr magneton number, and improves the B-B magnetic superexchange.