The ionic transport and dielectric behaviors in NaNbO3 were studied under pressures up to 29.1 GPa by in situ impedance spectroscopy measurements. The transport process consists of the ionic transfer and the Warburg diffusion process between sample/electrode diffusion layers. A dielectric relaxation with a giant dielectric constant at low frequencies is observed, which is attributed to the "Maxwell-Wagner" interfacial polarization. In the Pbcm phase, the increase in the interaction between the Na+ ions and the NbO6 octahedra results in the enhancement of vibration resonance damping. In the high-pressure phases, the decrease in the relative permittivity with pressure indicates the existence of space charge polarization of the interface layer besides the ionic polarization.