Structure and piezoelectric coefficient (d33) of lead-free 7.5% mole BaTiO3-doped (Bi0.5Na0.5) TiO3 (BNT-7.5%BT) polycrystalline piezoceramics have been characterized systematically as a function of poling electric (E) field. Dielectric permittivity and loss were also measured as functions of frequency and temperature. The piezoelectric coefficient d33 after poling at E=35 kV/cm can reach d33 similar to 186 pC/N, which is the highest value reported among (1-x) BNT-xBT compositions. A prior poling E field can reduce rhomobherdal lattice distortion, and enhance tetragonal phase and polarization ordering, that contribute significantly to the rapid raise of d33 and lower depolarizing temperature (Td). The reduced dielectric permittivity for the poled sample is attributed to ordered state and the pinning of field-induced nanodomain walls by the presence of oxygen vacancies.