Highly crystalline orthorhombic Bi2MoO6 particles with high visible-light photocatalytic activity have been controllably synthesized via a facile hydrothermal process without adding any surfactant. The morphologies of Bi2MoO6 with nanosheet and microrod can be selectively obtained by adjusting the pH value of the reactant. The formation mechanisms of nanosheet and microrod structures were then discussed based on the H+ cations adsorption abilities on different crystal faces. The Bi2MoO6 samples prepared at acidic condition showed 12 times higher photocatalytic activity than that prepared at basic condition under visible-light irradiation. The reason for the big difference in the photocatalytic activities for the Bi2MoO6 samples obtained at different pH values were systematically studied based on their shape, size and the variation of local structure.