Mg<sub>2</sub>(Si, Sn)-based thermoelectric materials, which are environmentally friendly and low-cost, have great development potential at moderate temperature range. Electronic transport properties of Mg<sub>2</sub>Si<sub>1-<em>x</em></sub>Sn<em><sub>x</sub></em> alloys can be optimized by doping elements. Doping is still one of the most effective methods for optimizing electronic transport performance, such as carrier concentration, mobility, and effective mass. The most effective doping elements are Sb and Bi. A significant amount of attention has been focused on the influence of element type and doping content. Different substituted sites will also have a great impact on the electronic transport parameters. In this work, the defect formation energy values of Mg<sub>2</sub>Si<sub>0.375</sub>Sn<sub>0.625</sub> alloys for Sb/Bi atoms substituting Sn/Si sites were calculated by first-principles calculations. The influence on electronic transport parameters was systematically analyzed combined with the calculated results of band structures and density of states. Corresponding component Sb/Bi doped Mg<sub>2</sub>Si<sub>0.375</sub>Sn<sub>0.625</sub> alloys were prepared by rapid solidification method, and microstructures, Seebeck coefficients, and electrical conductivities of the alloys were measured. Combined with the predicted results by solving the Boltzmann transport equation, electronic transport performance was compared and analyzed. The results indicate that, both Sn and Si sites were equally susceptible to Sb and Bi doping, but the Si sites were preferentially substituted due to their lower ∆<em>E</em><sub>f</sub> values. Doped Bi atoms provided a higher electron concentration, and Sb atoms provided a higher carrier effective mass. Thus, the maximum <em>σ</em> value of the Mg<sub>2</sub>Si<sub>0.375</sub>Sn<sub>0.615</sub>Bi<sub>0.01</sub> alloy was 1620 Scm<sup>-1</sup>. The maximum <em>S</em> value of the Mg<sub>2</sub>Si<sub>0.365</sub>Sn<sub>0.625</sub>Sb<sub>0.01</sub> alloy was -228 μVK<sup>-1</sup>. Correspondingly, the highest <em>PF</em> value for this alloy was 4.49 mWm<sup>-1</sup>K<sup>-1</sup> at <em>T</em>=800 K because the dominant role of <em>S</em> values. Although its power factor was slightly lower, the Mg<sub>2</sub>Si<sub>0.375</sub>Sn<sub>0.615</sub>Sb<sub>0.01</sub> alloy was expected to exhibit lower lattice thermal conductivity due to the lattice shrinkage caused by Sb substituting Sn sites. The optimal doping concentration of the Bi-doped alloys was lower than that of the Sb-doped alloys. These results are expected to provide a significant reference for the experimental performance optimization of Mg<sub>2</sub>(Si, Sn)-based alloys.
