Air pollution from particulate matter produced by incomplete combustion of diesel fuel has become a serious environmental pollution problem, which can be addressed by catalytic combustion. In this work, a series of K-modified MnO_δ catalysts with different microstructures were synthesized by the hydrothermal method, and the relationship between structure of the catalysts and their catalytic performance for soot combustion was studied by characterization techniques and density functional theory (DFT) calculations. Results showed that the prepared catalysts had good catalytic performance for soot combustion and could completely oxidize soot at temperatures below 400 °C. The cryptomelane-type K_2−xMn₈O₁₆ (K-OMS-2) with tunnel structure had excellent NO oxidation capacity and abundance of Mn⁴⁺ ions (Mn⁴⁺/Mn³⁺ = 1.24) with good redox ability, and it demonstrated better soot combustion performance than layered birnessite-type K₂Mn₄O₈ (K-OL-1). The T₁₀, T₅₀, and T₉₀ temperatures of K-OMS-2 were 269, 314, and 346 °C, respectively. The K-OMS-2 catalyst also showed excellent stability after five catalytic cycles, with T₁₀, T₅₀, and T₉₀ values holding in the ranges of 270 ± 2, 316 ± 2, and 348 ± 3 °C, respectively.