A polymorphic phase transition effect has been employed to regulate the piezoelectric property of lead-free (K,Na)NbO3-based materials, which intrinsically bear the disadvantage of the evident temperature sensitivity. In this research, Li-0.04[(K0.49Na0.51)](0.96)NbO3-xNa(2)B(4)O(7) piezoceramics were prepared through a conventional solid-state reaction. A piezoelectric coefficient d(33) of about 285pC/N and a unipolar strain of 0.14%@5kV/mm were achieved in the ceramics with x=0.5%. These improved properties can be ascribed to the coexistence of orthorhombic and tetragonal phase structure around room temperature. Besides, the field-induced unipolar strain varied less than 15% in the temperature region from room temperature to 180 degrees C. The stability of strain property could be rationally interpreted by a quantitative approach of electrostrictive coupling to the polarization amplitude.