Abstract: Photocatalytic performance was evaluated which was based on photocatalytic reduction of silver ion which was chosen as a probe in this work. Choosing the anatase titanium dioxide with grain size of 25 nm as the research object, the kinetics of photocatalytic reduction reaction were studied under UV irradiation and the influence on photocatalytic reaction rate of the concentration of the titanium dioxide and silver ion. The results showed that the reaction of the photocatalytic reduction of silver ions reflected the kinetics of the first-order reaction under the UV irradiation of 254 nm. The elemental silver was obtained through the reduction of silver nitrate with nano titanium dioxide. The optimal condition was the concentration of the nano titanium dioxide was 0.4 g/L and the concentration of the silver ion was 10～15 mg/L. Under this condition, the photocatalytic activity was examined by employing four totally different sizes of nano titanium dioxide. It has been proved that the silver ion was a suitable probe for photocatalytic reduction reaction.