A photocatalysis testing device includes a reaction chamber, a light source, and a spectrophotometer. Reactants including a methylene blue (MB) solution and a photocatalyst can be added to the reaction chamber and illuminated by the light source. Color vanishing rate of the MB solution can be measured by the spectrophotometer. Further, the photocatalysis testing device also includes a light-tight chamber and a temperature stabilizer. The reaction chamber and the light source are received in the light-tight chamber to avoid ambient light effecting on the photocatalysis test. The spectrophotometer is positioned outside the light-tight chamber and optically coupled to the reaction chamber. The temperature stabilizer is configured for stabilizing temperature of the MB solution.