A special seal is required at stern shaft, since it must serve the purposes of preventing both leakage of lubricating oil and seawater entering the stern shaft system. Improper design might cause leakage of fuel and pollution to the environment. Besides, the safety of machinery operation will also be affected. Ferrofluid seal is superior in terms of features such as long life, zero leakage, structure simplicity, fitting purposes of high-speed rotation, making it a new technology of sealing the stern shaft. In the following text, magnetic particles of Fe3O4 and solution of glyceride are made through proper chemical reactions, after which they are mixed as ferrofluid sealing liquid, and the performance of the liquid is studied experimentally. The experimental result indicates that magnetization intensity of the ferrofluid is increased as the intensity of outside magnetic field increases. Experimental instruments are designed for the purpose of research on ester magnetic fluid seal for stern. Through the experiment, performance of pressure resistance of esterified Fe3O4 ferrofliud is studied; also, main factors influencing pressure resistance of the studied ferrofliud are analyzed. The results indicate: pressure resistance decreases with increasing seal clearance or shaft rotation speed; whereby it is increased with the enhancement of saturated magnetic intensity of the ferrofliud.