In this study, the density of magnesium sulfate-based foam concrete was regulated by adjusting the foam dosage and the ratio of the foam stabilizer xanthan gum (XG) to the specialized foaming agent GX-7. The evolution of the pore structure was evaluated using bleeding rate tests and scanning electron microscopy (SEM). This investigation further elucidated the critical roles of density variation and pore morphology in determining the mechanical performance (compressive strength) and thermal insulation efficiency of the material. The results indicate that increasing the addition of high-stability modified foam significantly increased the pore density of the ultra-lightweight foam concrete while simultaneously reducing the average pore diameter. These microstructural changes led to a progressive decrease in bulk density, accompanied by a corresponding reduction in compressive strength. When the foam dosage reached 100% of the MgO mass, the material’s density decreased to 136.3 kg/m³, with a corresponding thermal conductivity of 0.081 W/(m·K). SEM micrographs revealed that, under these conditions, the pores exhibited a uniform morphology and well-defined structure, indicating an optimized pore architecture. However, when the foaming multiplier exceeded 125%, the frequency of bubble rupture increased markedly.