The invention provides a fuel tank tightness detecting device belonging to the technical field of machinery. The fuel tank tightness detecting device is used for solving the problem that a fuel tank tightness detecting device does not exist and comprises a bottom plate and a ventilation mechanism communicated with a fuel tank, an opened water tank is arranged on the bottom plate, a propping-pressing component propping against the fuel tank is arranged above the opening of the water tank, and a driving piece capable of driving the propping-pressing component to move so as to drive the propping-pressing component to press the fuel tank into water is arranged between the propping-pressing component and the bottom plate. The fuel tank tightness detecting device has the advantages of suitability of detecting various fuel tanks, convenience and rapidness, is flexible to use and is simple to operate.