Optical superlattice crystals were grown by the Czochralski method. The effect of thermal annealing below the Curie temperature on domain structures of a sample with good periodicity was studied. It was found that the periodic domain structure remained unchanged at annealing temperature lower that and began to deteriorate when annealed at above . A sample at for an hour almost changed to a single-domain structure except that a single-domain layer with reversed spontaneous polarization was formed at the edge of the sample. These results are useful for revealing the mechanism of formation of the periodic domain structure and designing a more favourable temperature field to improve the crystals' quality. A space-charge-field model was proposed to explain the phenomena.