The invention relates to the fields of oil-water separation and sewage processing, and specifically relates to a self-suction oil-water separating device having the advantages of high efficiency and low cost. The device comprises a liquid storage tank, a pipeline, a filtrate collection tank, a bushing type connector, a valve, a tapered tunnel, and a separation diaphragm. By utilizing a siphon acting force generated by difference between a high liquid level and a low liquid level, oil-water emulsion is automatically sucked to a separation membrane, the separation membrane, which has a special wetting performance, is used to demulsify the emulsion so as to separate the water phase and the oil phase. The problem that in the prior art, an oil-water emulsion (oil-in-water or water-in-oil emulsion) separation system has the shortages of high operation cost, bad continuity, low separation amount, and low efficiency is solved. The device has the advantages of simple structure, low cost, high separation efficiency, large separation amount, and good controllability, and can be widely used in the fields of oil-water separation and sewage processing.