The experiments and theoretical analysis on the erasure process of the grating, light-induced absorption and thermal activation demonstrated that there exists a shallow-level center besides a deep-level center in the Cu-doped KNSBN crystal. It was considered to be oxide vacancy formed during growth of the crystal. The gain of the two-wave mixing (TWM) reaches its maximum value when the temperature of crystal is about 360 K. The relationship between the gain of TWM and the crystal temperature was explained through two-center two-grating model. The function of the shallow-level center in photorefraction was discussed.