The (Pb(1-3x/2)Lax)(Zr0.5Sn0.3Ti0.2)O3(PLZST, 0.00≤x≤0.18)antiferroelectric ceramics were prepared by traditional ceramic process. The effect of La modification on phase structures, phase transition, and electrical properties of PLZST ceramics were investigated by X-ray diffraction, P-E hysteresis loop and dielectric spectroscopy. With the increase of La content x,the samples at room temperature are rhombohedral ferroelectric phase (x≤0.0.04),tetragonal antiferroelectric phase(AFEt, 0.06x≤0.0.08, and cubic phase (PE' cx≥0.1), respectively. The dielectric spectroscopy shows that with the increase of La content x, AFE-PE phase transformation temperature decreases, and for samples with La content x exceeding 0.12, strong dielectric frequency dispersion is observed around dielectric ε' and tanδ peaks. The peak temperatures Tm corresponding to the maxim dielectric constant shifts to higher temperatures with the increase of frequency, indicating that PLZST antiferroelectric transform to typical relaxors. The curves of dielectrics as functions of temperatures in the PLZST systems are fitted by a modified Curie–Weiss law proposed by Santos.