<p>Mg₃Sb₂-based thermoelectric materials have attracted considerable attention due to their non- toxicity, ease of fabrication, and outstanding performance in medium-to-low temperature ranges, yet a critical challenge remains in further enhancing electrical properties while effectively suppressing thermal conductivity. This study employs a Y-doping strategy combined with first-principles calculations to reveal the optimization mechanisms: Y incorporation shifts the Fermi level toward the conduction band, enabling n-type conductivity with a maximum carrier concentration of 1.26 × 10²⁰&#xa0;cm⁻³ and concurrently reducing the band gap to achieve a power factor of 14.82&#xa0;μW&#xa0;cm⁻¹&#xa0;K⁻²; the substantial mass disparity between Y and Mg induces mass-field fluctuations, triggering an avoided crossing phenomenon in the phonon spectrum that weakens acoustic-optical phonon branch interactions and significantly reduces phonon group velocity, thereby suppressing lattice thermal conductivity. Ultimately, the thermoelectric figure of merit reaches 1.056 at 725&#xa0;K. Crystal orbital Hamilton population (COHP) analysis further demonstrates strengthened Y–Mg bonding, which enhances structural stability. This work provides insights into dopant-mediated thermoelectric performance optimization through the coupling of electronic transport enhancement and phonon engineering.</p> Graphical abstract <p>By doping foreign elements, the Fermi level shifts from the valence band to the conduction band, thereby optimizing carrier concentration, as shown in Figs. (<b>a</b>) and (<b>b</b>). In addition, introducing mass differences between dopant atoms and matrix atoms drives the avoidance of crossover behavior between the acoustic and optical branches, as shown in Figs. (<b>c</b>) and (<b>d</b>). This phenomenon reduces phonon group velocity, as shown in Figs. (<b>d</b>) and (<b>e</b>), thereby suppressing lattice thermal conductivity and enhancing thermoelectric performance.</p>