Nearly all bacteria require iron for growth and survival. In aerobic environment, ferric iron almost cannot be utilized directly by bacteria. But ferrous iron is mainly existing in the host gastrointestinal. Soluble ferrous iron [Fe(II)] is imported directly via membrane transporters into periplasmic, then ferrous iron is imported via ferrous iron transport systems into cytoplasm. Most of gram-negative bacteria uptake ferrous iron by Feo system. The Feo transport system in Escherichia coli consists of the feoA, feoB, and feoC genes. In addition to the Feo transport system, the Yfe transport system and the Efe transport system and the Sit transport system are involved in the transport of ferrous iron. In this review, we described the component and functional mechanism of Feo system in gram-negative bacteria, to provide reference for studying other bacteria ferrous iron transport mechanism.