Alkaline K poisoned and Co-modified catalysts were prepared using Fe and Mn as active components, nickel foam as a carrier, and Co as a trace additive. The de-NO_x performance of the 3Mn10Fe/Ni catalyst under alkaline K poisoning conditions and the effect of Co modification on K resistance were studied. The results show that after the addition of K, the synergistic effect between Ni and Mn/Fe weakens, and the redox capability decreases, due to the decrease in NiO/Ni, Fe³⁺, Mn⁴⁺, lattice oxygen ratio, unsaturated Ni atom content on the catalyst surface and structure defects of Ni–Mn/Fe composite oxides. Brønsted acid sites decrease significantly, which decreases the low temperature de-NO_x activity of the catalyst. The NiO/Ni ratio, Fe³⁺, Mn⁴⁺, and lattice oxygen content increase after Co modified the 0.3K3Mn10Fe/Ni catalyst. The synergistic effect between Ni and Mn/Fe increases, and the redox capability is also promoted. Owing to the significant increase in Brønsted acid sites, the K poisoning resistance of the 3Mn10Fe/Ni catalyst also increases dramatically. When the Co addition amount is 0.2%, the optimal K resistance property of the 3Mn10Fe/Ni catalyst was achieved, and its de-NO_x activity reached 98% at 220 °C.