Molybdovanadophosphoric heterpoly acid H7[P2Mo17VO62]×39H2O was prepared from NH4VO3, NaH2PO4 and Na2MoO4 step by step, and characterized by IR spectrometry. Based on a model pollutant, Rhodamine B (RhB), the photocatalytic ability of H7[P2Mo17VO62]×39H2O was studied under UV-Vis light. The results showed that when H7[P2Mo17VO62]×39H2O was not added in the solution of RhB, the degradation rate of RhB was only 17.5% under UV-Vis light irradiation. Under UV-Vis light irradiation, the degradation rate had greatly improved while the concentration of prepared heteropoly acid was low in the solution of RhB, and the degradation rate increased with increasing of concentration of the heteropoly acid. And the degradation rate of RhB reached up to 75%. The photocatalytic degradation mechanism of RhB with Dawson type H7[P2Mo17VO62]×39H2O was also analyzed.