In this research, (1 − x)PLZST-xBiYO3 (x = 0–0.05) was prepared by a traditional solid-state process. The space structures of (1 − x)PLZST-xBiYO3 samples changed from hexagonal symmetry for x < 0.01, to monoclinic symmetry for x ≥ 0.01. Dielectric permittivity increased first and then decreased with the increase in BiYO3 content. The relaxor behavior of PLZST was disrupted slightly by BiYO3 doping. Then the local defects were created by BiYO3 doping, which formed weak couplings. The permittivity and maximum polarization were improved with the increase of BiYO3 content. When x = 0.01, Pm achieved the maximum of 9.02 μC/cm2 with an energy efficiency of 86.1%, which also exhibited the highest energy density, 0.679 J/cm3, in (1 − x)PLZST-xBiYO3 ceramics.