With the annual decline in drilling potential for adjustment wells, the measures currently adopted by mature oilfields, such as increasing water injection pressure and hydraulic fracturing, while maintaining stable oilfield production, raise production costs and increase the difficulty of profitable development of the oilfield. The downhole oil-water separation injection-production technology, centered on multi-cup equal-flow phase permeability separator, enables most of ground gathering, injection, and oil/water treatment to be performed downhole. This transforms the conventional surface-subsurface to macro-circulation waterflooding with a localized subsurface factory-style development mode. Without drilling new wells, the well pattern density can be effectively doubled, making it economically feasible to exploit reservoirs with water cuts exceeding 98% that cannot be economically developed by conventional methods.This approach enhances recoverable reserves while substantially reducing fluid production to the surface and lowering operating costs. The technology demonstrates significant benefits in reserve addition, production enhancement, and cost reduction, yielding considerable developmental and economic benefits.