© 2020 by the authors. Licensee MDPI, Basel, Switzerland. In this work, we report the ultrahigh electromechanical coupling performance of NBT-6BT-KNN lead-free single crystal at room temperature. The thickness mode electromechanical coupling coefficient (kt) and the 31 mode electromechanical coupling coefficient (k31) reach 69.0% and 45.7%, respectively, which are superior to the PZT-5H lead-based ceramics of kt ~ 60% and k31~39%. In addition, the evolution of the crystal structure and domain morphology is revealed by Raman scattering spectra, a polarizing microscope and piezoelectric force microscopy characterization.