A composition-induced pseudocubic–tetragonal structural transition was found to be accompanied by a relaxor phase transformation in xBi(Mg 0.5 Ti 0.5 )O 3 –(0.75−x)PbTiO 3 –0.25(Bi 0.5 Na 0.5 )TiO 3 ternary solid solutions. Dielectric and ferroelectric measurements suggest the coexistence of ergodic and nonergodic relaxor phases within a single pseudocubic phase zone for samples with 0.50<x<0.51 where large electromechanical strains of up to 0.43% (S max /E max =621pm/V) can be generated. The mechanism was mainly ascribed to the accumulated effects of field-modulated continuous and reversible transformations from a pseudocubic ergodic phase to a rhombohedral short-range ordered phase (but not nonergodic polar phase), and finally to a long-range ordered ferroelectric tetragonal phase. These procedures were found to be strongly dependent on the applied field magnitudes. These findings were reasonably approved by a couple of measurements such as dielectric–temperature–frequency spectrum, ferroelectric polarization/strain hysteresis loops, polarization current density curves and particularly ex situ Raman spectrum and in situ high-resolution synchrotron X-ray diffraction.