In spite of a strong visible light absorbance as far as 600 nm, CaNbO2N generally exhibits poor photocatalytic activity under normal conditions because of various structural defects and poor charge mobility. In this work, we have synthesized a series of Mg-modified CaNbO2N, i.e. CaNb1-xMgxO2+yN1-y (0 <= x <= 0.2), and performed a detailed investigation on their crystal structures, optical absorption and other physicochemical properties. Our results show that there is a slight shrinkage of the unit cell and a blue-shift of absorption edges upon Mg incorporation into CaNbO2N. The nitrogen contents as well as defects levels can be effectively tuned by altering the content of Mg. More strikingly, photocatalytic oxygen productions are much improved after Mg modifications under visible light irradiation (7%. 420 nm). An average oxygen production rate as much as - 126.8 umol h(-1) and an apparent quantum efficiency as high as similar to 3.4 % at 420 +/- 20 nm is achieved for CaNb0.9Mg0.1O2+yN1-y (x = 0.1). These improvements probably stem from a substantial decrease of Nb4+ defects in CaNbO2N as well as slight positive shift of valence band maximum (N/BM) after Mg modifications. Meanwhile, photoelectrochemical analysis suggests charge migration is somewhat enhanced in response to Mg modifications.