Chemical modification of MXenes, as an effective strategy to improve the electronic harmony with other materials, paves huge impetus to their practically industrial application. In this study, by means of Density Functional Theory (DFT) calculations, the adsorption and substitution of single transition metal atoms (M= 3d (Fe, Co, Ni, Cu, Zn), 4d (Ru, Rh, Pd, Ag, Cd), 5d (Os, Ir, Pt, Au, Hg)) on Ti3C2 MXene have been systematically investigated. It is found that the adsorption (adsorption energies E-ad = -1.05 similar to -7.98 eV) of single metal atoms on Ti3C2 are much stronger than those on graphene and graphyne. Interestingly, via a series of electronical structure calculations, we found good linear correlation between E-ad and chemically properties (such as the average bond distances d(M-Ti), the bader charge and the d-electron centre of metal), shedding light on the harmonic effect of doping metals both electronical and geometrically. In other words, the stronger adsorption, shorter bond distance, more bader charge, less negative d-electron centre. While for the substitution of single metal atom on Ti3C2, a linear relationship is also found except that the volcano curve are formed between the substitution energies and bader charges. Moreover, the charge transfer decrease firstly and then increase in both first and third layer. Generally, single-atom adsorption or substitution is a feasible method to improve the functionalization Ti3C2 MXene.