The magnetic alloy film has the advantage of high loss in the GHz range of microwave, but its magnetic permeability decays rapidly with the increase of thickness. The work aims to discuss the reason of magnetic decaying with the thickness of magnetic alloy films, and develop a low-cost method to maintain high magnetic losses under the condition of large film thickness. With PET as the substrate, FeNiCo films with high magnetic loss were prepared by electroplating, and magnetic metal stacked films with insulating interlayer structure were obtained by stacking the as-prepared alloy films with insulated substrates. The experimental results showed that when the thickness of the PET interlayer was 5 μm, the thickness of the FeNiCo magnetic film was 1.7 μm, and the total thickness of the stacked 10 magnetic films was 67 μm, the imaginary permeability at 1 GHz was as large as 40.69, and the value of reached 2 726. Moreover, the measured S21and Ploss/Pin of [FeNiCo/PET]10 by the micro-stripline method at 1 GHz were −9.0 dB and 0.83 dB, which were much better than the −1.9 dB and 0.58 of traditional magnetic composites with a thickness of 60 μm. Therefore, the insulation interlayers can effectively suppress the eddy current effect between magnetic films, so that it can maintain relatively high magnetic loss at a very large thickness, and achieve high electromagnetic noise suppression performance than existing materials.