Abstract: The research of submarine hydrothermal circulation and geological processes at mid-ocean ridges is a hotspot of international mid-ocean ridge research. Most of seafloor hydrothermal systems are directly related to submarine basalt and sub-seabed water-rock reaction. And, a new hydrothermal system is characterized by the ultramafic hosted hydrothermal systems with high concentration of H₂ and CH₄, and low concentration of SiO₂, which is located at slow-spreading Mid-Atlantic Ridge and ultraslow-spreading Gakkel Ridge in Arctic Ocean and Southwest Indian Ridge. Hydrothermal fluid composition, structure setting and sulfide mineralization of ultramafic hosted hydrothermal systems are different from basalt hosted hydrothermal systems. Ultramafic rock is generally exposed in slow-ultraslow spreading ridge, and high concentration of H₂ and CH₄ in vent fluid and high Co/Ni ratio in sulfide are the significant characteristics of ultramafic hosted hydrothermal systems. The discovery of ultramafic hosted hydrothermal systems has enriched the research contents of seafloor hydrothermal systems at the global mid-ocean ridge system, which is significantly important to the study of the international mid ocean ridge geological process, submarine hydrothermal activity and mineralization.