Sm-doped TiO2 photocatalysts were prepared by doping samarium ions into TiO2 nanoparticles in a sol-gel process. The samples were characterized using transmission electron microscopy (TEM) and ultraviolet-visible diffuse reflectance spectroscopy (UV-vis DRS). Their photocatalytic activities were evaluated by photodegradation of methyl orange (MO) in water under UV or stimulant solar light irradiation. The kinetics of MO photodegradation was further studied using multi-variable grey model. The experimental results indicated that the Sm3+ doping reduced the crystallite size, and appropriate doping induced red-shift of the absorption profile of TiO2. It was also found that the red-shift and absorptivity decreased in the range of 350-500 nm with the increase of Sm3+ doping and the optimal doping concentration was 0.05 mol%. Furthermore, the effects of Sm3+ doping concentration, the calcinating temperature of the samples, and the illuminating light on the photocatalytic activity of TiO2 were well simulated using multi-variable grey model.