The adsorption behavior and the separation efficiency of photogenerated electron-hole pairs are two important elements in estimating the photocatalytic activity of a photocatalyst. In this work, we have developed a facile one-pot solvothermal method for the preparation of Mo-doped Bi2WO6 with uniform three-dimensional (3D) hierarchical porous biscuit-like microstructures (PBMs). Mo doping is found to have two important roles in the synthesis of Bi2WO6 particles, leading to porous microstructures and adjusting band gaps of the Bi2MoxW1-xO6 particles. The band structure of the as-prepared porous Bi2MoxW1-xO6 products is characterized by UV-vis diffuse reflectance spectroscopy and valence-band X-ray photoelectron spectroscopy. Density functional theory (DFT) calculations give further insights into the band structure of the Bi2MoxW1-xO6 products. In all the samples, Bi2Mo0.21W0.79O6 PBMs exhibit a very efficient catalytic performance in oxidizing water under visible light irradiation (λ > 420 nm), with an average O2 evolution rate of up to 147.2 μmol h-1 g-1 and an apparent quantum efficiency (QE) of 3.1% at 420 nm, representing a 2 times more enhancement compared with the non-doped Bi2WO6 sample. This study provides a simple method for designing metal-doped semiconductors with porous structures for different applications.