In this paper, we report the synthesis of three kinds of novel nanosheet hierarchical cobalt particles by adjusting the [C₄H₄O₆]²⁻/Co²⁺ ratio through a liquid reduction method. We investigated the electromagnetic properties of the cobalt particles in detail over the microwave frequency range of 1–18 GHz. The results show that the real part of the permittivity decreases and the imaginary part of the permeability increases with an increase in the [C₄H₄O₆]²⁻/Co²⁺ ratio. The permeability displays two resonance peaks over the frequency range. The cobalt particles with [C₄H₄O₆]²⁻/Co²⁺ = 6 have a maximum reflection loss of −48.03 dB at 13.61 GHz, and the effective absorption bandwidth (RL ≤ −10 dB) is 6.76 GHz corresponding to a thickness of 1.7 mm. Considering the impedance matching and attenuation based on the electromagnetic parameters, we designed a way to obtain cobalt particles with excellent microwave absorption properties by decreasing the real part of permittivity and increasing the imaginary part of permeability.