A photoreactive polyimide, bearing a cinnamate photoreactive moiety in its side group, was synthesized. The photochemical reactions of the PSPI in the films were investigated in detail by UV-Vis and IR spectroscopies; the cinnamate chromophores were confirmed to undergo photodimerization. Linearly polarized ultra-violet light irradiation induced anisotropic reorientations of both the polymer main chains and the side groups in the film. The PSPI thin films were found to have unidirectional orientation ability as a result of photo-exposure with linearly polarized ultraviolet light, with the photoaligned polymer chains in the PSPI films able to effectively induce the alignment of nematic liquid crystals.