Photoelectrochemical (PEC) solar water splitting over oxynitrides is a promising process for renewable hydrogen production. However, the oxynitride heterojunction photoanodes with high charge-separation efficiency and stability, which have unique dimensionality-dependent integrative and synergic effects, are intriguing but still underdeveloped. Here, we design and fabricate the 1D/2D nanorod/nanosheet-assembled tantalum oxynitride (TaON) photoanode with the high PEC activity. Especially, integrated 3D heterojunction photoanodes comprising the 1D/2D barium-doped TaON (Ba-TaON) array and 2D carbon nitride (C3N4) nanosheets decorated with CoOx nanoparticles as a novel stack design were firstly prepared and the 3D CoOx/C3N4/Ba-TaON photoanodes with the remarkable photostability reached the pronounced photocurrent of 4.57 mA cm-2 at 1.23 V vs. RHE under AM 1.5G simulated sunlight. More broadly, the harness charge transfer process of this unique 3D heterojunction photoanode with the intrinsic requirements has been identified by the quantitative analysis combined with the electrochemical impedance and photoluminescence analysis. All the results highlight the great significance of the 3D dimensionality-dependent heterojunction as a promising photoelectrode model for the application in solar conversion. The cooperating amplification effects of nanoengineering from composition regulation, morphology innovation and heterojunction construction provide a valuable insight for creating more purpose-designed (oxy)nitride photoelectrodes with highly efficient performance.