In most reactor safety analysis system codes based on two-fluid models, the mechanisms of heat and mass transfer between the wall and the fluid depend on the flow regime map within the computational cell, which is associated with the average void fraction. However, when discontinuities in void fraction occur within the cell (e.g., due to the water level), the flow regime map may fail to accurately represent the actual flow conditions. Furthermore, under conditions of significant variation in void fraction distribution, the finite volume method with fixed Eulerian grids struggles to accurately predict the convection of mass, momentum, and energy between adjacent cells, leading to local pressure spike issues. To address these limitations, this paper proposes a water level tracking model based on grid tracking. Unlike the fixed grid size design in RELAP5, this model dynamically tracks the water level position to adjust the size of the momentum grid, effectively overcoming the deficiencies of the fixed Eulerian grid method. Validation using the Fill-Drain and Manometer cases demonstrates that the proposed model effectively resolves the local pressure oscillation problems caused by sudden changes in void fraction distribution, providing an innovative modeling approach for the development of thermal–hydraulic system codes.