Abstract：Research purposes： Disengaging of monolithic track bed caused by shield tunnel settlement has become an important factor affecting the safety of subway operation. To reveal the mechanical characteristics of monolithic track bed under shield tunnel settlement, a spatial coupling finite element model for a track - track bed - segment lining system is established. Based on the segment settlement monitoring data of Beijing Subway Airport Line T2, the deformation and failure characteristics of monolithic track bed are analyzed under disengaging of it.  Research conclusions： (1) When wave trough of local settlement at the expansion joint occurs in segment, fractures appear on the surface of the track bed at the settlement starting point, as well as two sides of the track bed bottom. When settlement trough is located in the middle of two expansion joints, a large scale of cracking phenomenon appears on the surface of track bed at the settlement starting point, while cracks appears on both sides of the bottom of the track bed and the track bed at settlement trough. (2) When the segment is in continuous settlement, and two subsidence troughs are not located in the expansion joint, the crack will appear at the bottom of a large range of bed surface, and tension stress is relatively small, without cracking. When the trough of one segment settlement area is located at the expansion joint, there will be cracking on both sides of the track bed, and a wide range of cracking on the surface of the 〖HJ〗ballast. (3) The research achievement has a certain value of reference for the maintenance in the shield tunnel of metro. 