In this paper, we reported the electrical fixing results of photorefractive index gratings in Cu- and Mn-doped KNSBN crystals at room temperature. Due to the higher Curie temperature of KNSBN, the fixed grating may have a longer lifetime than SBN crystals, and the fixing results are different from the observation in SBN crystals. The relation between the applied field and the revealed grating is studied by analyzing the domain fixing process. We also reported the comparison of the fixing results in the single domain crystals and the multi-domain crystals. In the multi-domain crystals, the diffraction efficiency of the revealed grating is as about three times as that of the recorded grating. The probable reason is also discussed.