Developing two-dimensional (2D) photocatalysts based on traditional layered materials provides new opportunities for photocatalytic applications. However, unlike van der Waals layered structures, the recently synthesized Bi2WO6 monolayer with unsaturated atoms showed more diverse photocatalytic performances. Here, a systematic study about the electronic and photocatalytic properties of monolayer, double-layer and bulk phase (MP, DP and BP) Bi2WO6 was carried out with first-principles calculations to disclose the origin of the superior photocatalytic activities of 2D Bi2WO6. In both MP and DP, holes and electrons manifested more effective transfer on account of the separate distribution and fast electrons export when compared with that in BP. Meanwhile, obvious structure reconstruction in MP resulted in a similar band gap with notable downward band edges when compared with that of BP, whereas DP indicated the highest conduction band minimum (CBM). As a result, the highest oxidising and reducing powers were expected in MP and DP, respectively. The maintained visible light absorption, reduced recombination of carriers and improved redox power in MP and DP boosted their photocatalytic activity synergistically when compared with that of BP. The present study is helpful for understanding and improving the photocatalytic activity of 2D Bi2WO6 as well as for exploring other non-van der Waals layered photocatalysts with atomic thicknesses.