By using frequency and time domain dielectric spectroscopy, the polar phase transitions of (Ba1-xSrx)TiO3 ceramics have been studied. Compared with the phase transitions of a ferroelectric crystal, the motion of space charges in ceramic has been obtained as well as lattice softening, excitation of screen charges and motion of ferroelectric domains. Slow effect, which is contributed by ferroelectric domains and space charges, can affect the accurate measurement by frequency domain method. In this paper we describe this influence and explain some experimental results. The reciprocal of frequency domain dielectric constant has a linear softening in the transition of primary structure, but has a parabolic type softening in that of high structure.