Cuprous oxide(Cu2O) microcrystals were successfully synthesized via hydrothermal method using CuSO4•5H2O and Na2SO3 in HAc-NaAc buffer solution. The products were characterized by X-rays diffraction (XRD) ,scan election microscopy (SEM) and ultraviolet-visible (UV-vis) spectroscopy. The photocatalytic activity of the as-prepared Cu2O powders was evaluated by the photocatalytic degradation of methyl orange solution under visible light irradiation at room temperature in air. The effects of chlorine ion on the photocatalytic activity of the Cu2O powders were investigated and discussed. The results show that chlorine ion in solution can obviously decrease the photocatalytic activity of Cu2O. When the concentration of chlorine ion increased to 0.5mol/L(about Cl- concentration in the seawater), the photocatalytic ratio of Cu2O decrease to 45% compared with the value in pure water. Increasing solution pH can enhance the photocatalytic activity of Cu2O. The mechanism of photo-degradation was discussed.