The oil-gas reservoir flow units have been widely used in the characterization of high, medium and low permeability reservoirs, but they are still rarely involved in the study of ultra-low permeability reservoirs. As to a case of the ultra-low permeability reservoirs of the Fuyu oil layers in the Fourth Member of the Lower Cretaceous Quantou Formation in Sheng 554 fault block of Sanzhao Sag in northern Songliao Basin, three types of flow units have been classified, including E type, G type and P type. The flow capacity is gradually reduced from E type to P type. The physical and spatial distribution characteristics of the flow units indicate that the flow units reservoirs are highly heterogeneous. The controlling effects of sedimentary facies and open normal faults on the development of flow units are analyzed. It is found that the flow capacity of the flow units themselves has little effect on the reservoir development, and the difference of the reservoir development effect caused by the difference of the flow capacity among different flow units is not obvious in the scale of ultra-low permeability reservoirs. The development effect mainly depends on the development factors such as the thickness of the perforated sandbody and the effect of water injection, and the geological factors such as the fault seepage channel, the mudstone seepage barrier and the thickness of sandbody. This is quite different from the previous view that the more the flow capacity of flow units is, the better the reservoir development effect of water injection is. The result is expected to provide some reference for the study of flow units in ultra-low permeability oil-gas reservoirs and for the development of ultra-low permeability reservoirs.