The electrochemical property of solid electrolyte plays a key role in stabilizing and enhancing the performance of all solid Li-ion battery, which means numerous effort is necessitated to explore and design better solid electrolytes. In this context, density functional theory calculations (DFT) were employed to investigate the electronic structures and ionic transport properties of NASICON M Ti2(PO4)3 ( M = Li, Na) materials aiming to elucidate the fast-ionic conductivity mechanism. The calculation results demonstrated that during the M ion migration, the Li/Na ions exhibit in both vacancy assisted and interstitial hopping, while the interstitial Li/Na diffusion with activation energies of 0.25eV for Li and 0.49eV for Na , is the kinetically favorable transport mechanism in their thermo-dynamically equilibrated configurations. However, the appearance of the interstitial M ion is strongly related to the ionic defect states and temperature, which indicates in real condition, two kinds of diffusion mechanism exhibit synergistic effect on the ion transport to realize the fast ion conducting in M Ti2(PO4)3 ( M = Li, Na) materials.