Ferroelectric phase coexistence was constructed in (1-x)BaTiO3-xCaSnO(3) lead-free ceramics, and its relationship with the piezoelectricity of the materials was investigated to ascertain potential factors for strong piezoelectric response. It is found that the addition of CaSnO3 caused a series of phase transitions in the (1-x)BaTiO3-xCaSnO(3) ceramics, and a ferroelectric coexistence of rhombohedral, orthorhombic, and tetragonal phases is formed at x=0.08, where the ceramics exhibit the lowest energy barrier and consequently facilitate the polarization rotation and extension, resulting in the optimal piezoelectricity of d(33) and k(p) values of 550pC/N and 0.60, respectively. Our study provides an intuitive insight to understand the origin of high piezoelectricity in the ceramics with the coexistence of multiferroelectric phases.