Abstract: The separation defects of monolithic track bed in shield tunnels have seriously affected the safety of sub? way operation. In order to study the law of separation defects of monolithic track bed, taking an operating subway line in China as the engineering background, it establishes a numerical model of monolithic track bed-segmentgrouting layer. The research results show that: (1) when the train wheel set acts on one side of the track bed near the expansion joint, it will cause larger separation deformation of the track bed, which is the most unfavorable position;(2) the separation amount caused by train load under vibration is much greater than that under static load, and there is a functional relationship between the two separations; (3) the location of the expansion joint is the area where separation is most likely to occur. The separation first occurs at the edges of both sides of the expansion joint, and with the increase of the action time of the dynamic load of train, the separation area extends to 1.5 m of the expansion joint; and (4) both the grouting layer stiffness and the contact surface bond strength are important factors affecting the separation amount, and increasing the stiffness of the grouting layer and the bond strength of the track bed-segment contact surface can effectively reduce the separation amount.