Cation Mg‐Dominated Coherent Phonon Transport Leads to Anomalous Thermal Conductivity in Mg₃Bi₂

N ‐type Mg 3 (Sb,Bi) 2 system has emerged as a groundbreaking thermoelectric (TE) material, renowned for its optimized electronic transport properties and intrinsically low lattice thermal conductivity (κ l ). However, the origin of anomalously low κ l and its weak temperature scaling (κ l ∼ T −0.5 , deviating from the classical T −1 behavior) remain elusive. Employing a theoretical framework that integrates ab initio anharmonic lattice dynamics into a unified heat transport theory accounting for particle‐like propagation (κ p ) and wave‐like coherent (κ c ) transport, phonon renormalization, and fourth‐order anharmonic phonon scattering, the thermal transport property of Mg 3 Bi 2 is comprehensively investigated. It is found that the weakly bonded interlayer cation Mg atoms amplify phonon coherence through dynamic disorder, driving wave‐like transport contributes ≈40% to κ l at 600 K, while four‐phonon scattering significantly suppresses particle‐like propagation (>40% reduction). The theoretical predictions of both the magnitude and temperature‐dependent behavior of κ l in Mg 3 Bi 2 exhibit synergistic agreement with in‐house and reported experimental values. This work pioneers key insights into the Mg cation sublattice tuning the particle‐wave duality in phonon transport, its strong anharmonic potential disrupts phonon phase coherence while enhancing four‐phonon scattering, collectively driving the low κ l in Mg 3 Bi 2 . This discovery provides two actionable design strategies—cation site engineering and coherent phonon tuning—for developing high‐ zT Mg 3 Bi 2 ‐based thermoelectrics.