We demonstrate the construction of heterojunction arrays for boosting solar water splitting by combining in situ guided growth of heterointerfaces and energy band tuning. By directly growing an ultrathin Fe2WO6 layer on a preformed WO3 nanosheet array in full coverage, a uniform and dense array of intimately contacted WO3/Fe2WO6 heterojunction was created. The heterojunction array shows not only a largely broadened visible light absorption range but also a built-in interface polarization to accelerate hole transfer from WO3 to Fe2WO6. Meanwhile, fine-tuning to match energy levels at the heterojunction was achieved by doping WO3 with Fe (Fe-WO3), leading to improved electrical conductivity and reduced charge recombination. Photoanodes based on such heterojunction arrays have shown significantly enhanced photoelectrochemical (PEC) water splitting performance, clearly arising from the above-mentioned efforts. Furthermore, by decorating FeOOH/NiOOH cocatalysts on the heterojunction arrays in tandem, the surface water oxidation kinetics was considerably accelerated, and the resulting Fe-WO3/Fe2WO6/FeOOH/NiOOH photoanode achieved a photocurrent density of 2.78 mA/cm2 at 1.23 V vs reversible hydrogen electrode. This work highlights the benefits of in situ construction of heterojunction arrays for enhancing the PEC performance.