With first-principles calculation of density functional theory, we investigate influence of interstitial atoms Zn, Nb and O on photoelectric characteristics of ZnNb₂O₆ system. It shows that lattice distortion of the defect structure is related to atomic size. However, as defect structure is formed due to electronegative differences of the interstitial atoms, length of bonds in the system changes. Systems containing interstitial atoms Zn and Nb show characteristics of a degenerate n-type semiconductor. Nb interstitial atom system shows a strong dielectric effect. By contrast, the O interstitial atom system, which is characterised by a degenerate p-type semiconductor, barely contributes to photoelectricity. It shows that the Nb interstitial atom system has potential in practical application.