0.25 wt% CuO-doped (Li,K,Na)(Nb,Ta)O3–AgSbO3 lead-free piezoceramics with pure perovskite structure were successfully prepared at a sintering temperature below 1000 °C. The sintering temperature of KNN-based piezoceramics was effectively reduced by about 100 °C due to the enhanced densification process induced by the addition of CuO. Besides, the acceptable sintering temperature window was broadened by the addition of CuO. It is found that the CuO-doped samples show slightly higher tetragonal–orthorhombic phase transition point (TT−O) but a lower Curie point (Tc), compared to undoped ones. The KNN-based piezoceramics became “hard” as CuO was added, supported by an increase of Qm. Fairly good electrical properties of d33*=383 pm/V, εr=860, Qm=188 and Tc=215 °C could be obtained in dense CuO-modified KNN-based piezoceramics sintered at 970 °C, demonstrating promising potential in practical applications.