Abstract: In petroleum equipment, ever if "o" sealing rings have been widely used, it is considered to be the user’ s headache for the high friction.But built-up sealing device, a newly developed one has the advantage of the"O"type and gets rid of its disadvantage.The sealing principle of this sort of device is described with the introduction of its structure.Analysis is performed on the pressure developed on the sealing ring or "O" type sealing ring of the device in the course of sealing with the theory of mechanics of material and Herz contaction theory, finding out the factor which causes the device to develop friction thus working out the equation for calculating the friction caused by the device, with theorectical instruction provided for the sealing device design.