With copper acetate as copper source, beta cyclodextrin as surfactant, NaOH as additive and KBH4 as reducing agent, the cuprous oxide nanoparticles with a diameter of about 50 nm were prepared in aqueous solution at room temperature. The composition, energy gap and photocatalytic performance of the products were analyzed. The study showed that the products had uniform particle size distribution, good crystallinity and high yield. As the as-prepared Cu₂O nanoparticles had a larger surface area, the catalytic efficiency on methyl orange was about 94 . 9 % in 30 min, almost all methyl orange was degradated, showing that the product had a good photocatalytic activity.