AgSbO3 was doped into KNN-based lead-free piezoceramics with an optimized composition of Li0.02(Na0.53K0.48)0.98Nb0.8Ta0.2O3 (abbreviated as LKNNT) to further enhance its piezoelectric property. The doping of AgSbO3 was found to be effective in reducing the grain sizes, resulting in more uniform microstructure in AgSbO3-doped LKNNT ceramics. AgSbO3 lowers tetragonal-orthorhombic phase transition point (TT-O), but with a more gentle rate as compared with other dopants. A large converse piezoelectric coefficient d33* up to 598 pm/V under a relatively low electric field of 1 kV/mm was obtained in the LKNNT-5 mol% AgSbO3 composition, whose tetragonal-orthorhombic phase transition point (TT-O) was controlled near room temperature, but its Curie temperature was kept at 235°C. The d33* obtained in the present material is a very high value for nontextured KNN-based ceramics, which is attributed to the polymorphism phase transition effect and “soft” behavior caused by the addition of AgSbO3.