Abstract: Addressing the frequent issue of segment flotation during shield tunnel construction, this study first employs a self-developed testing device to reveal the time-dependent variation patterns of net buoyancy in typical single-component grout and two-component grout. A universal mathematical expression for net buoyancy versus time is established using the ExpDec2 model. Subsequently, by comprehensively considering grout buoyancy dissipation characteristics, segment self-weight, shield thrust, ground load, and tail restraint, a finite element model for segment flotation calculation is developed using ABAQUS. This model elucidates the flotation deformation behaviors of segment linings under typical single-component and two-component synchronous grouting and is validated against field monitoring data. The results show that: (1) The time-dependent variation of net grout buoyancy can be divided into two phases: a linear slow-decreasing phase (Phase Ⅰ) and a nonlinear rapid-decreasing phase (Phase Ⅱ). Notably, the net buoyancy diminishes to zero while the grout remains in a fluid-plastic state, far from its final setting time. (2) The proposed segment-grout layer computational model effectively captures the flotation characteristics of segments encapsulated by gradually gelling grout, showing good agreement with field measurements. This model can serve as a theoretical basis for flotation-resistant design in similar projects. (3) Compared to single-component grout, two-component grout exhibits significantly shorter buoyancy dissipation time and physical setting time, enabling rapid stabilization of segments and substantial suppression of flotation. Hence, for projects with severe segment flotation, it is recommended to adopt synchronous grouting materials with fast gelling time and high early-stage strength.