Abstract: Photosystem Ⅰ (PSⅠ) is a pigment_protein complex embedded in the photosynthetic membrane, which includes more than ten protein subunits, and catalyses the transfer of electrons from the PC to the Fd through a series of electron transfer components. There is considerable progress in the structural and functional study of PSⅠ in the past two decades, and especially in recent years. In this review, we describe the protein composition of PSⅠ and its characteristics, the three electron transfer processes that PSⅠ mediates, the unique light harvesting pigment_protein complex (LHCⅠ) of PSⅠ and the latest 4*! resolution three_dimensional structural biology information of PSⅠ. We also foresee the future research of PSⅠ.