Without the assistance of a WO3 seed layer, a uniform tungsten trioxide hydrate (WO3·H2O) plate-like array film was grown directly on bare fluorine-doped tin oxide (FTO) glass by a simple hydrothermal method at mild temperature using ammonium oxalate ((NH4)2C2O4) as a structure-directing agent. The dependence of the crystal structure and morphology on the growth temperature and growth time in the as-prepared samples was studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The XRD studies showed that the as-prepared thin films obtained below 150 °C were comprised of orthorhombic WO3·H2O and were completely converted to monoclinic WO3 at 180 °C. SEM analysis revealed that the thickness of the WO3·H2O nanoplates increased with the increase of growth temperature as well as growth time. Moreover, the formation mechanism of the WO3·H2O plate-like arrays was discussed. It was found that the (NH4)2C2O4 played an important role in the formation of vertically aligned plate-like arrays. The thin films calcined at 450 °C for 1 h showed fine photocatalytic activities. The plate-like arrays grown on the bare FTO substrate synthesized at 120 °C for 12 h exhibited the best photocatalytic activity, which generated an anodic photocurrent of 4.13 mA cm−2 at 1.6 V (vs. Ag/AgCl) under illumination of a 500 W Xe lamp in 0.5 M H2SO4 electrolyte.