The utility model relates to an online leakproofness detection device of over -and -under type check valve, include with valve body (110) threaded connection of over -and -under type check valve (100) false valve gap (200), be used for pouring into the pressure medium into power device (300) in false valve gap (200). Install on the over -and -under type check valve false valve gap (200), and the medium through vent line (221) blowout -valve intracavity gas, when the system pressure is the same when valve cavity pressure is online with the valve, is closed the medium and is injected pipeline (211) and vent line (221) valve into after passing through medium injection pipeline (211) admission valve intracavity, forms airtight space in the valve pocket, and case (130) and disk seat (120) become sole leakage point at this moment. Pressurize 5min, observe the pressure of precision pressure gauge (420) this moment, if valve cavity pressure reduces, explains then to exist and leak that valve seal nature is unqualified, if do not reduce in the valve pocket, then explanation nothing is leaked, and valve seal nature is qualified.