Abstract: Following applicable product specification, hydrostatic test is required for steel tubes to ensure the quality. The conventional hydro-tester is bothered with such shortcomings as rather low productivity, high operation and maintenance costs, and untested section of tube end, etc. Elaborated here in the article is a newly-developed tube end sealing device for the high-pressure hydrostatic testing machine, being applicable to the dual- line hydro-tester. The device features such a function, i.e., once the tube is fully filled with hi-pressure water, the sealing ring is deformed under the water pressure, and thus gets self-sealed. The said sealing device is suitable for tube O.D. sealing in case of testing with extra-high water pressure.