It is important to characterize photocatalysts by in-situ techniques. In this work, the high pressure temperature-programmed in-situ Fourier Transform Infrared Diffuse Reflection Spectroscopy (FTIR-DRS) was employed to evaluate shallow trap states in Cu-doped TiO2 photocatalyst. It was demonstrated that the formation of the shallow trap state by reduction is much easier for CuO/TiO2 than pure TiO2. Furthermore, the formation temperature of the shallow trap state for CuO/TiO2 is the same as its starting reduction temperature of Cu2+ to Cu+, which indicates that the generated shallow trap state might be Cu+.