Although the pronounced piezoelectricity was obtained in (K, Na)NbO3 piezoceramics with the phase boundary engineering (PBE), the physical mechanisms remain pending. Here we revealed for the first time how PBE influences the piezoelectric properties through synergetic contributions. Cryogenic experiments confirm that PBE constructs a phase coexistence, consisting of rhombohedral (R), orthorhombic (O), and tetragonal (T) phases, with a structural softening, by which a high piezoelectric coefficient d33 of 555 pC/N and the enhanced temperature stability of strain are achieved. The phenomenological theory and transmission electron microscope demonstrate that the superior d33 hinges on the flattened Gibbs free energy and the abundant nano-domains (10-80 nm), which respectively induce the enhanced permittivity and the coexisting single-domain and multi-domain zones. In particular, we disclosed a trade-off relationship between ferroelectric domains and polarnanoregions (PNRs) and found the “double-edged sword” role of PNRs in the piezoelectricity enhancement. Therefore, this work helps understand the physical mechanisms of the piezoelectricity enhancement, benefiting the future research of lead-free piezoceramics.