Two-dimensional (2D) Janus structure is promising for photocatalytic water splitting because its intrinsic built-in electric field can overpass the bandgap requirements of photocatalysts. However, how to achieve overall water splitting spontaneously in one structure is still difficult. Here, we present a novel family of 2D Janus perovskite oxynitrides, BaXNO (X = Ta, Nb, V), with a large intrinsic built-in electric field based on first-principles calculations. Both 2D BaTaNO and BaNbNO exhibit remarkable activity for photocatalytic overall water splitting. For BaNbNO, a lowest overpotential of 0.44 V for the oxygen evolution reaction (OER) is realized. The Gibbs free energy of hydrogen adsorption (ΔG) is near zero (0.02 eV) for BaTaNO in the hydrogen evolution reaction (HER). Interestingly, 2D BaVNO shows in-plane ferroelectricity due to the displacement of V ions accompanied by a ferroelastic distortion of the lattice, which modulates the band structures, resulting in optimized band alignment. The photocatalytic performance can be further improved by constructing a BaNbNO/BaVNO heterostructure, which has an overpotential of 0.47 V for the OER and ΔG of 0.16 eV for the HER. Our theoretical investigation provides not only a guidance to design Janus photocatalysts for full water splitting, but also strategies to modulate the photocatalytic performance through ferroelectricity.