TiO$_2$ nanotube arrays prepared by anodization technology are modified with a samarium ion (Sm$^{3+}$) by a hydrothermal method for use in photodegradation of methylene blue (MB). The samarium ion as a down-shifting luminescence material can improve UV radiation harvesting to increase visible light utilization. The efficiency of the photocatalytic activity for the modified-TiO$_2$ nanotube arrays in degradation of MB was investigated under UV–Vis light irradiation. The results show that Sm–TiO$_2$ nanotubes can increase the photocatalytic efficiency of MB. When TiO$_2$ nanotubes are modified by 0.02 M Sm$^{3+}$, MB can be almost completely degraded when compared with bare TiO$_2$ nanotubes. This indicates that TiO$_2$ nanotubes structure, surface area and good UV radiation harvesting play important roles in the degradation of MB.