Fundamentally understanding lattice dynamics and thermal transport behavior in liquid‐like, partially occupied compounds remains a long‐standing challenge in condensed matter physics. Here, the microscopic mechanisms are investigated underlying the ultralow thermal conductivity in ordered/liquid‐like Cu ₃ BiS ₃ by combining experimental methods with first‐principles calculations. The ordered structure and liquid‐like are first experimentally synthesized and characterized, partially Cu‐atom occupied Cu ₃ BiS ₃ structure with increasing temperature. Selfconsistent phonon calculations are then combined, including bubble‐diagram corrections, with the Wigner transport equation, considering both phonon propagation and diffuson contributions, to evaluate the anharmonic lattice dynamics and thermal conductivity in phase‐change Cu ₃ BiS ₃ . The theoretical model predicts an ultralow thermal conductivity of 0.34 W m ⁻¹ K ⁻¹ at 400 K, dominated by diffuson contributions, which accurately reproduces and explains the experimental data. Importantly, the machine‐learning‐based molecular dynamics (MD) simulations not only reproduced the partially Cu‐atom occupied Cu ₃ BiS ₃ structure with the space group Pnma but also successfully replicated the thermal conductivity obtained from experiments and Wigner transport calculations. This observation highlights the negligible impact of ionic mobility arising from partially occupied Cu sites on the thermal conductivity in diffuson‐dominated thermal transport compounds. This work sheds light on the minimal impact of ionic mobility on ultralow thermal conductivity in phase‐change materials. It demonstrates that the Wigner transport equation accurately describes thermal transport behavior in partially occupied phases with diffuson‐dominant thermal transport.

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