The spin-orbit interaction of light refers to the interaction between the spin angular momentum and the orbital angular momentum of light, which exists in basic optical processes such as reflection, refraction, scattering, diffraction, and focusing. In traditional large-scale classical optics, the influence of spin-orbit interaction can be ignored, but at sub-wavelength scales, strong coupling between spin and orbital angular momentum occurs. This article reviews the basic origins and important applications of the spin-orbit interaction of light. Firstly, two important basic concepts of photon spin-orbit interaction, photon angular momentum and geometric phase theory, are introduced. Secondly, two types of spin-orbit interactions, spin-intrinsic orbit interaction and spin-extrinsic orbital angular momentum interaction, are briefly introduced respectively. Then, the research progress and representative applications of photon spin-orbit interaction are mainly introduced. Finally, the challenges and future research directions of the research on the spin-orbit interaction of light are prospected.