The global distributions and structure characteristics of mid-ocean ridges were introduced in this paper, including the classification of main mid-ocean ridges based on spreading rates, basic characteristics of mid-ocean ridges such as geomorphology features, the relationship between crust thickness and spreading rate, the magma chamber beneath the mid-ocean ridge as well as the interaction between the plume and the mid-ocean ridge. The applications and some important achievements of the ocean bottom seismometer (OBS) in surveys on the mid-ocean ridges were reviewed.