Abstract: The wellhead water injection pressure is lower than 31.4 MPa for deep reservoirs of low and extremely low permeability, little water is injected into the well which leads to low production speed and large natural descending. Study on water injection technology of extremely high pressure is conducted, which increases the injection pressure approximately up to the formation fracture pressure, so that micro-fractures can be produced and thus improve the permeability and push the water into formation, so as to compensate formation energy and improve the flooding efficiency. The study coves injection methods, well selection principle, casing protection, surface engineering auxiliary technology, safety operation procedures and designing specifications, etc. The technology has been applied for 3 years and remarkable economical and social benefits have been achieved.