The utility model provides an offshore domestic sewage treatment system comprising a regulating tank and a membrane treatment tank, wherein the regulating tank is connected with an offshore domestic sewage inlet pipeline and a seawater inlet pipeline and is internally provided with an electrolysis device; the membrane treatment tank is connected with a clear water outlet pipeline and is internally provided with an MBR (Membrane Biological Reactor) membrane module; the membrane treatment tank is connected with the regulating tank through a regulating tank water outlet pipeline. The treatment system provided by the utility model is capable of combining an electrolysis method and a membrane separation method, good in stability, high in reliability and small in size and can be widely applied to an offshore platform and a ship. By using the treatment system provided by the utility model, the offshore domestic sewage can be treated effectively, the COD (Chemical Oxygen Demand) concentration in the drained water can be reduced to be less than 30mg/L, and the IMO (International Maritime Organization) standard requirement can be met.