Organic content and the pollution of natural water can be characterized by membrane pressure and membrane area in membrane pressure method. Effect of the molecular structure of organic pollutants on their surface membranes formed on the air/water interface was studied using π-A isotherms,Fourier transform infrared spectroscopy and Atomic force microscopy. It was shown that the number of hydroxyl groups had great effect on the properties of surface membranes. At the same temperature,the collapse pressure of the lipid with 3 hydroxyl groups was higher than that of the lipid with 1 hydroxyl group because of stronger hydrogen bonding. In addition,more condensed surface microstructure was found for the former lipid at the same surface pressure. The results may provide a basis for the establishment of the state equation of surface membrane and the measurement of surface membrane of urban sewage.