The first-principles calculation was used to investigate the influence of doping fourth-period transition metal elements on the structural, mechanical, and thermal properties of Mo₂CoB₂. Through the calculation of cohesive energy and formation enthalpy as well as the calculation comparison between the obtained results and Born-Huang criterion, all doped compounds are thermodynamically and mechanically stable. Point defect theory was employed to determine the occupation sites and occupation preference of doped elements in the Mo₂CoB₂ crystal cell. Results show that Sc and Ti exhibit strong preference for Mo sites, and V has a weak preference for Mo sites. Additionally, Cr, Mn, Fe, Cu, and Zn have a weak preference for Co sites, and Ni has a strong preference for Co sites. Debye temperatures were obtained by the contrast calculation. The results reveal that except Mo₇TiCo₄B₈, Mo₇VCo₄B₈, and Mo₇CrCo₄B₈, the doped models all have lower Debye temperatures than the undoped model, suggesting that except Ti, V, and Cr elements, the addition of transition metal elements of large quantity into the Mo₂CoB₂ hard phase should be avoided. Furthermore, except that of the Cr-doped model, the hardness of the doped models is lower than that of the undoped model, and the models with doped elements at preferential sites normally exhibit higher hardness than those at non-preferred sites do. This research provides theoretical basis for the development of Mo₂CoB₂-Co cermet with improved properties.