The invention discloses a detecting device for detecting the sealing separation volume of a flange sealing element. A displacement transferring plate is arranged between an equipment flange and an end cover flange, the displacement transferring plate and an end face of the equipment flange are hermetically arranged via a sealing element, the sealing element is in contact with one end face of the equipment flange to form face sealing or linear sealing, one end face, adjacent to the end cover flange, of the displacement transferring plate is provided with a displacement detection point which is consistent with axial displacement of a point on the intermediate diameter of the sealing element on the same cross section, a detecting assembly is arranged to detect the axial displacement of the displacement detection point, and a detection result is acquired by a data acquisition system, so that the sealing separation volume between the flange and the sealing element can be obtained conveniently, and a research basis is provided to research of relationship between the sealing separation volume and the sealing leakage volume of the flange sealing element. A detecting method is simple, convenient and high in operability.