Abstract: A series of ammonium salts of 12-tungstophosphoric acid (NH₄)_xH_3-xPW₁₂O_40，（x = 0.5, 1.0, 1.5, 2.0, 2.5, 3.0) were synthesized by the precipitation method at room temperature. The resulting materials were characterized by XRD, FT-IR, DR UV-Vis, SEM, and N2 adsorption-desorption. Their catalytic activities in the green synthesis of benzoic acid were evaluated under organic solvent-free conditions. The results suggested that the substitution of H⁺ by NH₄⁺ did not change the Keggin structure of H₃PW₁₂O₄. On the other hand, the particles became more uniform and the lattice constant decreased with the increase of x in (NH₄)_xH_3-xPW₁₂O₄₀. Meanwhile, the specific surface area and pore volume increased to some extent and (NH₄)_2.5H_0.5PW₁₂O₄₀ exhibited the maximum values. It was interesting that (NH₄)_xH_3-xPW₁₂O₄₀ showed higher catalytic activities than that of bulk H₃PW₁₂O₄(71%), and the yield of benzoic acid may reach up to 76% over (NH₄)_2.5H_0.5PW₁₂O₄₀