Abstract: Downhole oil-water separation and rejection through same well technology is that liquid production is separated under the hole.After that,the oil concentrated liquid is lifted to the surface,while the separated water is injected into another formation.It has two kinds system,one is hydrocyclone separation system,and the other is gravity separation system.The principle and applicability of both system were discussed,and their well claim was introduced.The experiment of the split ratio determination of gravitational oil-water separation had been done and the result indicated that the split ratio of good separation effect could be less than 0.7 when the production is less than 50m3/d.So the conclusion that the experiment method is helpful to study the working status of complex downhole oil-water separation system can be given.In the same time,it illustrated the technology’s advantage and disadvantage,pointed out the research direction in the future.