During high-temperature crystal growth, lattice defects will inevitably form inside piezoelectric materials, which can be a hindrance for performance optimization. Through appropriate atmosphere control during sintering, defect levels inside the piezoelectric material can be regulated. Herein, CaZrO3-modified (K, Na)NbO3-based lead-free piezoelectric ceramics with a nominal composition of 0.95(Na0.49K0.49Li0.02)(Nb0.8Ta0.2)O-3-0.05CaZrO(3) are produced by sintering in an oxygen-rich atmosphere. Compared with an air-sintered sample, the piezoelectric constant of the oxygen-sintered sample has greatly improved 15% up to 390 pC/N, which is comparable to commercial lead-based counterparts. In addition, the planar electromechanical coupling factor k(p) is enhanced from 0.46 to 0.52. A qualitative model related to defect engineering is proposed to support the experimental observations. Our results indicate the feasibility of purposely optimizing the piezoelectric performance by sintering atmosphere control.