The ferroelectric hysteresis loops of 63PbTiO3-37BiScO3 ceramics are measured under sinusoidal electric field varying from 5 to 55 kV/cm in a frequency range from 0.1 to 100 Hz. The fitting results show that the logarithm of remanent polarization and the logarithm of coercive electric field are both linearly related to electric field in the first and third field regions, but not in the second region. The three-stage behavior is distinct from the existing two-stage behavior, and it can be attributed to the dependence of ferroelectric polarization behavior on field stage.