The diffusion mode of the non-steady-state photo-EMF excitation is investigated in periodically poled MgO:LiNbO₃ crystal at light wavelength λ=457 nm. The signal excitation is carried out along the polar axis of the crystal; the dependencies of the signal amplitude versus the phase modulation frequency, intensity and spatial frequency of the interference pattern are studied in this geometry. The analysis of these dependencies allows determining the photoelectric parameters of the material - the type, value and relaxation time of the photoconductivity, as well as the diffusion length of charge carriers.