In an effort to find high-energy-density and high-mobility cathode material for K-Ion batteries, this paper presents a systematic study of multiple doping concentrations of P3-type K0.5Mn1-xMxO2 (M = Fe, Ti, and Cu) materials using first-principles approach. The most thermodynamically stable phase of each material is predicted by the formation of mixing enthalpy, and the optimal doping concentration of each doping element is screened. It is found that the redox activity of K0.5Mn1-xMxO2 is derived from the anion redox of O2−/O1− during depotassium. At the same time, doped Mn-based cathode materials have the characteristics of large capacity, stable structure, good thermal stability and small diffusion energy barrier. Among them, Fe doping will promote the charge compensation of O 2p-electrons in the intercalation process and reduce the Jahn-Teller distortion caused by Mn3+ (ratio < 46.67 %). Ti doping can improve the lattice stability of O, inhibit the precipitation reaction of oxygen, make TiO more covalent, and reduce the structural change during depotassium. Therefore, the overall thermal stability of the material is improved. Cu doping reduces the diffusion barrier of the material (0.54 eV). Through the basic understanding of the redox reaction mechanism in K0.5Mn1-xMxO2, the work of this paper lays a foundation for the rational design of high-performance K-ion batteries.