The photorefractive grating in a KTa1-xNbxO3:Cu (x=0.32) crystal was erased with an expanded beam and one of the writing beams, respectively. Through the different erasing effects, it was further demostrated that besides a periodic space-charge field a DC space-charge field was formed simultaneously during writing. Meanwhile the dynamics of erasing a DC space-charge field was also studied. In addition, by irradiating KTa1-xNbxO3:Cu crystal with one of the writing beams under an applied electric field before writing, the influence of the formation of DC space-charge field on the diffraction intensity was investigated.