Abstract: Addressing the problem that there exist a gap between the outer shoulders of the inner and outer threads of the direct-connection casing during the connection operation,a method is introduced here in the essay to block the shoulder of the direct-connection casing by means of the sealing ring,focusing on the design of the structural parameters of the sealing ring.Thanks to the said design method,the outer shoulder gap of the direct-connection casing is eliminated so as to prevent any media intrude into the inside of the connection.The feasibility of the method is verified via relevant test.