[(Na1-xKx)(0.5)Bi-0.5]Ti0.985Ta0.015O3 (abbreviated as Ta-NK100x) lead-free ceramics with good piezoelectric properties were prepared using a solid-state reaction method. The structure and electrical properties of Ta-NK100x had been systemically investigated. The highest bipolar strain of 0.458% and the unipolar strain 0.448% are achieved at x=0.18 at 60kV/cm. Meanwhile, the corresponding normalized strain reaches 747pm/V. In addition, the unipolar strain of the poled Ta-NK18 increases to 0.537%, and corresponding d33 increases slightly to 894.5pm/V at 60kV/cm. The electric-field-induced phase transition between ferroelectric and relaxor is found to play a dominant role in the origin of the large strain. Moreover, the strain behavior remains stable within 10(5) switching cycles which indicating the prepared ceramics are promising candidates for actuators and stress sensors.