The ceramics of 0.96K0.48Na0.52Nb(1-x)TaxO3-0.04BaZrO3-0.3%MnCO3 were prepared via solid-state reaction and subsequent sintering in air and reducing atmospheres. The effects of Ta doping and sintering atmosphere on the microstructure and piezoelectric properties of ceramics were investigated. The results demonstrate that all the ceramics sintered in different atmospheres exhibit a pure perovskite structure. The phase structure of KNN-based ceramics is the coexistence of rhombohedral and orthorhombic phases. Also, the generation of oxygen vacancies is suppressed as Ta doping content is increased. When x = 0.1, the ceramics sintered in a reducing atmosphere exhibit the optimum electrical properties (i.e., d33=172 pC/N, d*33=294 pm/V, kp=24.9, εT33=820, and tanδ=0.021), which are superior to the counterparts sintered in air atmosphere (i.e., d33=142 pC/N, d*33=220 pm/V, kp=21.1, εT33=593, and tanδ=0.022). It is indicated that sintering in a reducing atmosphere can suppress the grain growth, thus facilitating the densification of ceramics and improving the piezoelectric properties.