In this paper, single-phase cementite powders are prepared by mechanical alloying and vacuum annealing heat treatment methods. Combined with first-principles calculations, the effects of Mn, Cr, and Si on the phase formation ability and magnetic properties of cementite are analyzed. It has been found that single-phase cementite can be obtained by mechanical alloying + 600 ° C vacuum heat treatment. The addition of Si completely inhibited the formation of cementite, while the addition of Mn and Cr could promote the formation of cementite. The saturation magnetization and coercive force of Cr and Mn alloyed cementite are lower than those of pure Fe3C when unalloyed, and the effect of the reduction range caused by the Cr element is greater.