In this paper, CdSe quantum dots (QDs) photocatalysts were successfully synthesized by a hydrothermal method. The factors affecting synthesized CdSe QDs were investigated under different experimental conditions. CdSe QDs were characterized by X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, transmission electron microscopy (TEM), ultraviolet visible diffuse reflectance spectrophotometry (UV-vis DRS), photoluminescence (PL) spectroscopy, and X-ray photoelectron spectroscopy (XPS). The photocatalytic activities of CdSe QDs were evaluated on the decomposition of cefalexin under UV irradiation. The results showed that the degradation ratio of Cefalexin could reach 70.34%. Based on the results, the possible mechanism of the photocatalytic reaction on cefalexin with CdSe QDs was discussed.