Owing to the relatively short hole diffusion length, severe charge recombination in the bulk of bismuth vanadate (BiVO₄) is the key issue for photoelectrochemical water splitting. Herein, we design a nanoporous MoO_3−x/BiVO₄ heterojunction photoanode to promote charge separation. The efficient electron transport properties of oxygen deficient MoO_3−x and the nanoporous structure are beneficial for charge separation, leading to a significantly enhanced PEC performance. The optimized MoO_3−x/BiVO₄ heterojunction photoanode exhibits a photocurrent density of 5.07 mA·cm⁻² for Na₂SO₃ oxidation. By depositing FeOOH/NiOOH dual oxygen evolution cocatalysts to promote surface kinetics, a high photocurrent density of 4.81 mA·cm⁻² can be achieved for PEC water splitting, exhibiting an excellent applied bias photon-to-current efficiency of 1.57%. Moreover, stable overall water splitting is achieved under consecutive light illumination for 10 h. We provide a proof of concept for the design of efficient BiVO₄-based heterojunction photoanodes for stable PEC water splitting.