In summary, CsCdPO₄ adopts a periodic 3D Cd–P–O anionic network at RT. As the temperature increases above 440 K, CsCdPO₄ retains the overall 3D Cd–P–O anionic framework but exhibits IM long-range ordered PO₄ arrangement. Structure analysis and theoretical calculation confirm that thermal vibration induced torsion and deformation of PO₄ are constrained by the 3D Cd–P–O network, which is critical for the unique periodic to IM long-range ordered phase transition of CsCdPO₄.