Although BaNbO2N has visible light absorption as far as 740 nm, its photocatalytic activity is generally very poor under ordinary conditions, being incommensurate to its light absorption properties. In this work, we introduce Ca into the B site of BaNbO2N to form a complex perovskite BaNb1-x/3Cax/3O2+yN1-y (0 <= x, y <= 1). The presence of Ca in BaNbO2N has great impact on a number of important properties such as band gap value, nitrogen content, surface hydrophilicity, and defects levels. In particular, defects such as Nb4+ species are effectively suppressed in Ca-modified BaNbO2N. More importantly, photocatalytic activity of BaNbO2N for water oxidation reactions is substantially improved after Ca modifications. Compared with pristine BaNbO2N, more than twofold enhancement in photocatalytic oxygen evolution is observed for BaNb0.8Ca0.2O2+yN1-y (x = 0.6). Such improvements probably stem from the suppression of Nb4+ species in the structure as well as enhanced surface hydrophilicity after Ca modification. The facile approach by incorporating alkaline earth cations into crystal structure can be well extended to other perovskite oxynitrides whose photocatalytic activity is subject to those defective species.