TonB systems of gram-negative bacteria play an important role in transportation of nutriment from outside environments. TonB systems consist of plasma membrane proteins ExbB-ExbD and periplasmic protein TonB, which provide the energy to TonB-dependent receptors to transport substrates. These substrates include iron, hemin, vitamin B12, carbohydrate and some transition metal elements. The energy supporting function of TonB relies on its special structure which contains N-terminal domain for fixation, flexible periplasmic linker Pro-rich domain and C-terminal domain for contacting receptors. The precise mechanism of TonB system is not fully understood though its structural was studied a lot. To provide insights into direction for further research of TonB, we reviewed the TonB-dependent substrates uptake, structural features, functional mechanism and expression regulation of TonB.