To greatly enhance the mechanical quality factor (Q(m)) of piezoceramic materials, B2O3-CuO mixed oxides were added to a K0.48Na0.52NbO3-based lead-free piezoceramic (abbreviated as BC-KNN). The results suggest that the B2O3-CuO additives effectively improved the sinterability and Q(m) value of the piezoceramic. An optimal Q(m) value as high as 2128 was obtained, which is 35 times higher than that of pure KNN ceramic. Interestingly, we found that the Q(m) value was sensitive to humidity of the surrounding environment. As the relative humidity (RH) increased from 25% RH to 78% RH, the Q(m) value of the BC-KNN ceramics decreased from 2128 to 267. We found that the dependence of the Q(m) value on humidity was closely related to the instability of the relative dielectric constant ((r)). Our results show that a dense microstructure is critical for maintaining a stable high Q(m) performance in a humid environment.