The lead-free (1-x)Bi4Ti3O12/xBi4.5K0.5Ti4O15 composites (x = 0–0.4) have been synthesized by solid state reaction method. Evolution of microstructure and electric properties has been investigated with varying composition ratio. X-ray diffraction, Raman and IR absorption confirm the crystal transition tendency from orthorhombic to tetragonal phase. Evident texture along c-axis crystallographic orientation is observed in structure and morphology characteristics with x = 0.05 and 0.1. Through modulating the mole ratio of Bi4.5K0.5Ti4O15, coexistence of ferroelectric order and relaxor state is verified by the variation of ferroelectric hysteresis loops and polarization current density. This behavior can be attributed to the uncertain stability of discernable ferroelectric order induced by electric field among polar nanoregions during symmetry distoration. Temperature dependence of dielectric properties suggests an excellent thermal stability during a wide temperature range. Moreover, the detailed dielectric mechanism has been analyzed via various methods. Electrical conduction in grains and grain boundaries respectively plays the different role during the varying temperature process. •(1-x)Bi4Ti3O12/xBi4.5K0.5Ti4O15 (x = 0–0.4) are synthesized by solid state reaction.•Crystal transition from orthorhombic to tetragonal phase can be observed.•Electric properties show a coexistence of ferroelectric order and relaxor state.•Electric response exhibits mixed conductivity mechanisms for varying temperature.