Herein, a sodium magnesium diphosphate, Na7Mg4.5(P2O7)(4), was synthesized via a standard solid-state reaction method, and a single crystal X-ray diffraction method was adopted to determine its crystal structure. It was found that it crystallizes in the triclinic system with the space group P (1) over bar. The crystal structure of Na7Mg4.5(P2O7)(4) features a parallelogram channel composed of corner/edge-sharing isolated [P2O7] dimers and a Na(2)O-5 polyhedra to form a 3D framework. Thermal behaviors indicate that Na7Mg4.5(P2O7)(4) congruently melts and IR spectrum shows the existence of the P2O7 groups. Moreover, Na7Mg4.5(P2O7)(4) possesses an experimental band gap of 5.90 eV and an ultraviolet (UV) cut-off edge of about 210 nm, which was confirmed via diffuse reflection spectrum. To analyze the contribution of the atomic orbitals to the energy band, band structures and partial density of states (PDOS) of Na7Mg4.5(P2O7)(4) have also been reported.