Rare earth phosphate has a wide application in optical materials, laser materials and many fields with great development prospects. In this paper, LaPO4 nanorods with different crystalline phases are prepared with hydrothermal method, its photocatalytic performance are discussed, as well as the structure and UV diffuse reflectance spectroscopy. The research indicates that the hydrothermal temperature plays a key role in the crystal phase, while it is hexagonal at 120 ℃, monoclinic at 180 ℃ and the mixed phase at 160 ℃. The UV absorption has no obvious change of each crystalline phase, and they all show nanorods. The photocatalytic activity of LaPO4 for the degradation of MB is studied; it is found that the monoclinic LaPO4 has the best photocatalytic activity. Furthermore, monoclinic LaPO4 could decrease the fluorescence quantum efficiency and increase the separation efficiency of electrons and holes, which improve the photocatalytic activity. Besides, the main active species are confirmed to be hydroxyl radicals with capture experiments during the photocatalytic process.