The invention discloses a device and a method for detecting the leak rate of an O type rubber sealing ring used for spaceflight, which belong to the technical field of the detection of spaceflight components. The device comprises gas storage equipment, an inflation valve, a first pre-pumping valve, a second pre-pumping valve, a leak detection tooling, a leak marking valve, a standard leak, a vacuum gauge, a deflation valve, a leak detection valve, a leak detector, a forestage angle valve, a prepump, a molecular pump, an ultrahigh vacuum gate valve and a vacuum booster pump. The method comprises the following steps of: firstly, pumping the pressure of a detection chamber and a dummy module below the air pressure of 10Pa, then pumping the detection chamber to a lower pressure by the molecular pump, forming a pressure difference between the detection chamber of the leak detection tooling and the dummy module and afterwards introducing high-purity gas with a standard atmosphere pressure into the dummy module. If a leak exists in a rubber sealing ring to be detected, the high-purity gas can enter the detection chamber by the sealing ring under the action of the pressure difference, and the leak rate of the sealing ring can be accurately calculated by the magnitude of the change of the output indication of the leak detector. The invention has the advantages of high detection sensitivity, high detection efficiency, reliable result and the like.