Abstract: The oil-water interface neutron gauge for oil storage tank is developed by using the neutron back scattering technique. The slow neutron detectors move down and up in the tank and scan dynamic ally the material in different height. On the basis of the different slow neutron counting rate of gas, oil and water, the three-phase interface heights are determined. At the same time, by making use of the counting rate curve the water ratio distribution curves are gained, and the total content of oil in the tank are determined as well.