Abstract: The Sn58Bi alloy plug, composed of 58% bismuth (Bi) and 42% tin (Sn), emerges as a promising alternative to conventional cement plugs. Investigating its mechanical behavior throughout the molten-to-solidified transition is crucial for predicting its performance in downhole oil and gas applications. Particular emphasis was placed on characterizing early expansion behavior during solidification, as the magnitude of expansion force directly correlates with sealing integrity. To analyze temperature and expansion force dynamics during plug formation, a specialized experimental apparatus was developed. Expansion and sealing integrity tests revealed three key findings: Applying overlying axial pressure (0–2 MPa) significantly enhanced plug sealing capacity, with a linear relationship observed between sealing performance and pressure magnitude. In addition, slower and more uniform cooling facilitated expansion both radially and at the axially constrained bottom. Increasing the length-to-diameter ratio (L/D) of 2 to 6 induced sequential solidification patterns, wherein final-stage solidification drove radial expansion of residual molten alloy, thereby improving gas sealing integrity. These findings establish a theoretical framework for the application of Sn58Bi alloy as downhole casing-plug material.