Perovskite oxynitrides often own a poor photocatalytic activity under normal conditions, being incommensurate to their strong visible light absorbance. This is particularly true for SrTaO2N which undergoes self-oxidative decompositions even under protection of a hole scavenger. In this work, we laminate the crystal structure of SrTaO2N by inserting extra layers of SrO to form a Ruddlesden-Popper (RP) compound Sr2TaO3N. This structural modification not only improves the light absorption of SrTaO2N but also effectively suppresses the defect formation such as Ta4+ species etc. More importantly, Sr2TaO3N is able to drive photocatalytic water oxidation reactions under visible light illumination (lambda >= 420 nm) without the aid of a cocatalyst and self-oxidative decompositions found for SrTaO2N are largely inhibited. Further analysis suggests that the presence of extra SrO layers positively shifts the valence band edge and stabilizes N species in the structure according to Pauling's second rule. Theoretical calculations indicate that Sr2TaO3N has typical 2D charge transportation properties which are associated with the structural laminations. Its conduction band minimum (CBM) and valence band maximum (VBM) are found to be located within TaN2O2 square planes which favors efficient charge transportations.