An outer pressure type expanding joint with an axial displacement detecting device comprises an inlet end pipe, an outlet end pipe, a protecting outer cover, a corrugated pipe and the axial displacement detecting device; the axial displacement detecting device is composed of a metal annular outer polar plate, a metal annular inner polar plate, a fixed assembly and a capacitance detector; the outerside of the metal annular inner polar plate is sleeved with the metal annular outer polar plate in a non-contact manner; the metal annular outer polar plate is fixed to the outer wall of the non-fixed end of the protecting outer cover through the fixed assembly; the metal annular inner polar plate is fixedly arranged on the outlet end pipe; the capacitance detector calculates and obtains an axialdisplacement value of the expanding joint according to a capacitance value between the metal annular outer polar plate and the metal annular inner polar plate in the displacement state; through the two sleeved annular polar plates, the axial displacement value of the expanding joint is calculated and obtained due to the capacitance value obtained due to detection; and the outer pressure type expanding joint with the axial displacement detecting device is simple in structure and can adapt to the work conditions of overground, pipe ditch and direct burying expanding joints and the like.