In this study, the phase diagram and electric properties were demonstrated for a (Mn, K)-modified Bi-0.5 Na0.5TiO3 (BNT)-based solid solution. (0.935-x)Bi0.5Na0.5 TiO3-xBi0.5K0.5TiO3-0.065BaTiO(3) with 0.5% mol Mn doping was prepared by a conventional solid-state reaction method. A morphotropic phase boundary (MPB) formed between the ferroelectric rhombohedral and tetragonal phases around x of 0.04 with the MPB tolerance factor t of 0.984-0.986. The temperature and composition dependence of the dielectric, piezoelectric, ferroelectric properties along with the strain characteristics were investigated in detail and a phase diagram was presented. Around the MPB region, the maximum values of piezoelectric constant d(33)* of 290 pC/N, d(33) of 155 pC/N, dielectric constant epsilon(T)(33)/epsilon(0) of 1059 and low dielectric loss tangent tan delta of 0.017 were obtained. In addition, the authors also suggest that the solid solution with composition x of 0.24, exhibiting both high-depolarization temperature T-d of 182 degrees C, d(33)* of 156 pC/N, d(33) of 130 pC/N, will be favorable for high-temperature actuator and sensor applications.