The current oil/water separation membrane researches based on special wettability are seldom involved in the dynamic process of the membrane separation and the relevant quantitative calculations. In this paper, by analyzing the physical quantities of the superhydrophilic membrane separation process based on the capillary mechanics and by classifying the membrane pores of membrane materials based on special wettability as capillary pores, a mathematical model is constructed, and it is used to analyze the force distribution condition in the separation process of oilwater mixture by using the superoleophilic membrane. Then, the principle of the superhydrophilic and superoleophilic membrane in the air transforming to be superoleophobic underwater is analyzed from the perspective of surface tension, and the capillary flow rate, the capillary permeation rate, the membrane flux and the critical penetration pressure in the membrane separation process are calculated based on capillary mechanics and are validated by experiments. On this basis, the separation mechanism of superhydrophilic and underwater superoleophobic oil/water separation membrane is preliminarily discussed based on capillary mechanics.