Precious metals (like Ir, Ru, and Pt) and their derivatives are the benchmark catalysts for water splitting in acidic media due to their high stability and activity. However, the high cost and scarcity of these materials hamper the large-scale applications. To solve this issue, construction of catalysts containing low content of precious metals with high intrinsic activity can be an efficient strategy, which expectedly can decrease the cost but meanwhile preserve the activity. Herein, we synthesized an IrW/WO₃ array catalyst by in situ formation of IrW alloy on hierarchical WO₃ nanosheet arrays. With extremely low Ir content of 1.25 wt % in 0.5 M H₂SO₄, this composite catalyst not only shows superior water oxidation activity (the overpotential at 10 mA cm^-2 is only 229 mV, significantly lower than that of the commercial IrO₂ (358 mV)) but also exhibits excellent proton reduction performance (the overpotential at -10 mA cm^-2 is 49 mV, close to that of commercial Pt/C catalyst (42 mV)), showing promising bifunctionality for the overall water splitting. As a result, only 1.5 V is needed to drive the overall water splitting at 10 mA cm^-2 with a good long-term stability under acidic conditions. These remarkable features can be ascribed to the abundant active sites exposed by the three-dimensional nanostructure, and the high intrinsic activity per Ir site. The theoretical calculation verifies that Ir sites in IrW surface after oxidation have a higher intrinsic activity than IrO₂ for water oxidation. We believe this research can supply a strategy to design highly active and stable catalysts with low loading of noble metals for acidic water splitting.