A mixed-crystal TiO2 was prepared by the sol-gel method for the degradation of the water pollutant metformin (MET) under ultraviolet light. The structure and photocatalytic activity of mixed-crystal TiO2 were explored by various characterization methods. The degradation mechanism of MET is revealed by degradation kinetics calculations, radical trapping experiments, and analysis of degradation products. SEM, TEM, XRD and FTIR showed that the TiO2 prepared at the optimal calcination temperature of 550 °C was a mixed crystalline structure, containing 77% anatase and 23% rutile, and the morphology was 20 nm spherical aggregation structure. The degradation experiments of MET showed that the effect of mixed crystal was better than that of single crystal, which was due to the oxygen vacancy/Ti3+ and narrow band gap of mixed TiO2. The MET degradation process conformed to the first-order kinetic model, and the degradation rate constant was 0.016 min-1. At the pH of 11, the degradation rate of MET reaches up to 78.03%, which is 2.2 times than anatase-type TiO2. Radical trapping experiments demonstrate that the degradation of MET follows a mechanism involving hole and ?OH、?O2? oxidation. Through the in-depth analysis of the degradation products, four degradation reaction paths were proposed. Furthermore, after five consecutive cycles, the catalyst exhibits only a 2.59% decrease in the degradation rate of MET, indicating its high photocatalytic activity and stability.