Bismuth molybdate photocatalysts with different phase structures and morphologies were controllably synthesized via a refluxing method by adjusting the pH in the reaction system. Bi2MoO6 nanosheets were easily obtained under acidic conditions, while Bi3.64Mo0.36O6.55 nanoparticles were formed in circumneutral and basic solutions. The mechanism for the formation and phase transition of these two bismuth molybdates is based on tuning the pH value, which can control the growth rate along different crystal axes and the formation of different hydrolysis products that act as the initial seeds of the crystallographic phases. The photocatalytic activity of the Bi2MoO6 nanosheets for MB and MO degradation was higher than that of the Bi3.64Mo0.36O6.55 nanoparticles under visible light irradiation, and the highest photocatalytic activity was observed for the Bi2MoO6 nanosheets prepared at pH 6.0. The high visible-light photocatalytic activity of the Bi2MoO6 nanosheets arises from the easy separation and transfer of photogenerated electron-holes in the nanosheet's structure as well as the narrow band gap, which leads to an improvement in the visible absorption ability. Electron spin resonance (ESR) and a photogenerated carrier trapping experiment suggested that both Bi2MoO6 and Bi3.64Mo0.36O6.55 had the same photocatalytic mechanism and the main oxidative species for these samples was the hydroxyl radical.