Achieving solid-state electrolytes (SSEs) that combine fast Na conduction, high-voltage stability, and deformability remains a formidable challenge for all-solid-state sodium-ion batteries (ASSNIBs). Here, we introduce a cyclic trimetaphosphate anion (PO ) as a transformative network-forming unit to construct a new family of amorphous oxyhalide SSEs via facile mechanochemical synthesis. The unique nine-oxygen-donor architecture of PO enables robust, three-dimensional coordination with metal chlorides (MCl, M = Zr, Ta, Hf, Nb, Al), forming a rigid yet disordered framework where isolated Cl anions are strategically liberated. This distinctive structure enables a dynamic anion‑assisted transport mechanism: the PO ‑bridged network provides stable conduction channels, while the mobile Cl anions dynamically assist Na hopping by mitigating steric and electrostatic barriers, collectively achieving an ultralow activation energy of 0.33 eV. The optimized electrolyte exhibits a high room-temperature ionic conductivity of 0.80 mS·cm and a wide electrochemical window of 1.4-4.2 V. ASSNIBs assembled with a NaNiFeMnO cathode demonstrate stable cycling at 4.2 V, retaining 92% capacity after 300 cycles at 0.5C. This work pioneers the use of macrocyclic polyphosphates in SSEs, establishing a new design paradigm that simultaneously addresses ionic conductivity, stability, and interfacial compatibility.