A new kind of 3D hollow aggregate spherical structure WO3 nano-plates was prepared via a simple hydrothermal synthesis without any template. In this paper, the effects of HNO3 addition on the preparation of WO3 powder were systematically investigated. With HNO3 concentration increasing, the product was converted from WO3(H2O)(0.33) to monoclinic WO3 center dot H2O until the reaction to complete. Meanwhile, the morphology and size of particles changed from mocro-scale spheres to nano-plates. In the WO3 center dot H2O structure, -H2O integrates the H2O molecules via hydrogen bond, while -WO3 bonds with other WO3 molecules and finally form a hollow spherical structure. During subsequent drying, WO3 center dot H2O is deprived of crystal water to become monoclinic WO3, remaining this porous spherical structure. Compared with the industry-use catalyst of Degussa P25 TiO2, the as prepared WO3 exhibits the wonderful photocatalytic activity due to the plate-like morphology and porous structure.