We studied the transport properties of short period AlGaN/GaN superlattice induced by terahertz field. Due to the strong polarize effect it is not precise enough to calculate the energy dispersion relation just using the nearest wells in tight binding method. The distortion of the conduction band profile by the polarization fields has been taken into account in our improved model. The calculations indicate that the potential wells of next to nearest neighbors cause anharmonic electron oscillations at the multiples of the fundamental Bloch frequency. The results show that the AlGaN/GaN superlattice is a promising candidate to convert radiation of frequency ω to radiation at frequency 3ω or even higher.