A series of cationic functional polymers with different cations and molecular weights were used to determine the oil water interfacial tension, Zeta potential and O/W emulsion stability of the oil and water of the Gudong crude oil. It was found that when the molecular weight was almost the same, the bigger of the cationic degree, the better ability of breaking the O/W emulsions by the polymers . And in the region of the investigated concentration, at higher concentration of the polymer, better demulsification effects were obtained. The results of interfacial tension and zeta potential measurements indicated that the reason of the demulsification of emulsion by this kind of cationic functional polymer was the electrical neutralization on the interface between oil and water. That is to say, when polymer was added to the emulsion, it could neutralize the charges on the interface and as a result the oil droplets coalescenced.