The “hopping” model is used to analyze the transient behaviours of fundamental harmonic of the space-charge field with external AC field. Exact and approximate dynamic analytical solutions are presented in the presence of sinusoidal AC field and square-wave field. For the applied AC field, it is found that the real part of the fundamental space-charge field varies at the same frequency as that the applied field does, the imaginary part oscillates at twice the frequency. The photorefractive grating is phase-shift for high frequency of external applied alternating field. These results are in agreement with those derived from the band -transport model.