A device for detecting the leak rate of a leak is employed to realize a detection method for detecting the leak rate of a leak. The device includes a gas storage tank for storing a detection medium, a connecting piece, a detection valve arranged on the connecting piece, a sealed container and a balance for measuring the weight of the sealed container. The device can detect the leak rates of leaks within the range of 1×10−1˜1×10−8 Pa·m3/s.