By studying the alone and synergetic effects of Zr4+ and Bi3+ on potassium sodium niobate ((K, Na)NbO3, KNN) ceramics, we revealed how Bi0.5A0.5ZrO3 (A = K, Na, Ag, and (Na0.82K0.18)) reduces the orthorhombic-tetragonal phase transition temperature (TO–T) of KNN ceramics. Investigations into the alone effects reveal that aliovalent substitutions on K+/Na+ (Nb5+) with Bi3+ (Zr4+) inevitably destroy long-range ordering (LRO) and thus worsen piezo/ferroelectric properties. Despite this, Zr4+ can replace Nb5+ within a high content, remain an orthorhombic (O) phase, and slightly increase TO–T. Although substituting on K+/Na+ with Bi3+ decreases TO–T, it already significantly destroyed LRO before shifting TO–T to room temperature. Then, investigations into the synergetic effects show that Zr4+ acts as a buffer, Bi3+ is an accelerator, and A+ is a stabilizer. The buffer can exist in KNN ceramics within a high content and neutralizes the charges caused by the accelerator that concentrates on decreasing TO–T, and the stabilizer compensates for the stability of the perovskite phase. Their synergetic effects explain why Bi0.5A0.5ZrO3 can gradually reduce the TO–T of KNN ceramics without significantly destroying LRO. Therefore, this work helps understand how Bi0.5A0.5ZrO3 decreases TO–T and further design the phase boundary for KNN ceramics.