K-doped FeOOH/Fe₃O₄ nanoparticles with a size of 100 nm were grown on stainless steel (SS) hydrothermally. The effect of the synthetic conditions on compositions, morphologies and electrochemical performances was discussed, and the possible synthetic mechanism was proposed. The obtained K-doped FeOOH/Fe₃O₄/SS demonstrated a superior specific capacity of 396 mA h g⁻¹ (0.554 mA h cm⁻²) at 1 A g⁻¹ in 2 M KOH, which is the largest value reported so far. And it was also much larger than those of K-doped and -undoped FeOOH/SS samples. Furthermore the sample manifested an increasing capacity during the cycling test, which was probably associated with the doping of K⁺ as well as the coexistence of Fe(II) and Fe(III) in the structure. Based on density functional theory (DFT), the effect of the K-doping on the electron conductivity of the sample was theoretically investigated. In addition, a Co–Mo–O/Ni₃S₂/Ni foam (NF)//K-doped FeOOH/Fe₃O₄/SS asymmetric supercapacitor was fabricated, which can deliver a high energy density of 74.38 W h kg⁻¹ at a power density of 3.64 W kg⁻¹.