Abstract: To investigate the mechanism of segment floating in shield tunnels, a simulation method based on the Penalty Immersed Boundary Method (PIBM) is proposed. The method uses the collocated grid Finite Volume Method (FVM) to discretize the flow control equations of the synchronous grouting layer, simulates the interaction between segments and grout via PIBM, describes the dynamic response of segments using rigid body motion equations, and applies the PISO algorithm to decoupling flow field pressure and velocity. This enables numerical solution of the fluid-solid coupling system of grout and segments. Comparisons with field monitoring data from two engineering cases, i.e., Hangzhou Metro Line 7 and Sophia Tunnel, demonstrate that the calculated segment floating displacements and the grout pressure distributions within the synchronous grouting layer agree well with measured data, with relative errors within 0.21% and 0.128%, respectively. The results validate the applicability of the proposed method. Furthermore, a simulation analysis of the segment floating process reveals that during the rapid floating stage, segments are driven upward by buoyancy. This forces the grout behind the lining to flow downward along the shield tail gap and accumulate at the bottom. The flow trajectory transitions from initial "stratified differential flow" to "overall directional flow," and the grout pressure exhibits a horizontally symmetric distribution with the minimum pressure at the top and the maximum pressure at the bottom. Compared with the conventional equivalent layer method, the proposed method dynamically captures the interaction process between grout and segments, visually presenting the real-time changes and distribution characteristics of the grout flow field in the synchronous grouting layer. This study provides a foundation for in-depth investigations into the mechanism of segment floating in shield tunnels.