Lead-free (0.99 − x)K0.5Na0.5Nb0.96Sb0.04O3-0.01BaZrO3-xBi0.5Na0.5ZrO3 (KNNS–BZ–BNZ) piezoelectric ceramics were fabricated by the conventional solid-state reaction method. The effects of the addition of Bi0.5Na0.5ZrO3 on phase structure, microstructure, and electrical properties were studied in detail. The rhombohedral–tetragonal (R–T) phase boundary was established in the ceramics with 0.03 ≤ x ≤ 0.04. Owing to the both R–T phase boundary and the enhancement of dielectric and ferroelectric properties, an optimal electrical property (e.g., d33 = 450 ± 5 pC/N, kp = 0.50 ± 0.02, and TC = 252 °C) was attained in the ceramics with x = 0.03. As a result, constructing phase boundary is proved to be a good way to improve the electrical properties in KNN-based ceramics.