The invention provides a testing device for fatigue crack growth rate under a seawater corrosive environment and a measuring method for the testing device. The testing device is characterized by comprising a three-point bending fixture and a seawater environment container tank, wherein the seawater environment container tank is connected with a circulating seawater supplying device positioned outside the seawater environment container tank. The testing device and the measuring method disclosed by the invention are suitable for measuring the fatigue crack growth rate of a single edge notch SE (B) sample; a conventional three-point bending fixture is inversely mounted, and a machining gap of the SE (B) sample and a compliance measurement COD (Crack Opening Displacement) gauge are enabled to be positioned above the sea level, so that the COD gauge can be measured by a compliance method and the condition that the COD gauge is corroded since the COD gauge is placed in seawater can be avoided; besides, by using an anti-splashing cover plate, the phenomenon that a fixture system and a fatigue testing machine are corroded due to seawater splashing during a fatigue test is prevented; the testing device has high practicality.