Lead-free piezoelectric ceramics of 0.94(K0.5Na0.5)NbO3–0.06LiNbO3 (0.94KNN–0.06LN) were prepared by an ultrasonic irradiation method. X-ray diffraction, thermal analyses, and X-ray photoelectron spectroscopy were used to evaluate the effects of different ultrasonic powers on the structural and electrical properties of the 0.94KNN–0.06LN ceramics. The results revealed that the ultrasonic irradiation facilitated the decomposition of starting carbonates, promoted the perovskite phase formation and the transformation from orthorhombic to tetragonal phase, and resulted in improved ceramic density. 0.94KNN–0.06LN ceramic samples with a relative density of 94% were obtained after the ultrasonic irradiation processing under air sintering, which exhibited a piezoelectric constant of 184 pC/N. This study showed that ultrasonic irradiation was an effective method for producing high-quality KNN-based lead-free ceramics with significantly reduced processing time.