Abstract: Water phase height is measured by the color step caused by the color differences between diesel and water at the two-phase interface, and a comparison is made between experimental value of water phase height and calculated results in the case of assumed oil-water interface being a plane to indicate that a certain deviation is available. Therefore, oil-water interface concave-convex in the case of blending of different oil-water volume ratios is analyzed based on assumed circular arc interface shape. There is a concave-convex transition point for the oilwater interface changing with the water phase volume fraction, and the corresponding water phase volume fraction is about 0.5. Geometrical models for calculation of oil-water interface curvature are given. Oil-water interface curvatures with different water holdups are obtained by the method of numerical resolution and critical water phase volume fraction corresponding to the oil-water interface contact angle under experiment conditions are predicted.