To explore the influence laws of different mix ratios on the physical and mechanical properties of phosphogypsum-based foamed building gypsum, in order to improve the resource utilization rate of phosphogypsum, the experiment adopted single-factor experimental analysis and microscopic morpho-logy detection methods to systematically study the physical and mechanical properties, thermal properties and pore structure of phosphogypsum-based foamed building gypsum under three different mix ratios. The microscopic morphology of phosphogypsum-based foamed building gypsum under different mix ratios and the interaction mechanism between each component were deeply observed and analyzed using scanning electron microscopy (SEM), revealing the intrinsic relationship between the microstructure and macroscopic performance changes. The test results showed that the apparent density and mechanical properties of phosphogypsum-based foamed building gypsum decreased with the increase of foaming admixture content; the optimal mix ratio was: phosphogypsum content of 60%, cement content of 20%, slag powder content of 10%, gypsum whisker content of 10%, retarder content of 0.1% of the cementitious material, water-to-solid ratio of 0.5. Correspondingly, the optimal sodium dodecyl sulfate (K12) foaming agent content was 12.5%, and the phosphogypsum-based foamed building gypsum prepared with this foaming agent reached the optimal level of absolute dry strength; SEM tests indicated that gypsum whiskers filled the pores between crystal connections, increasing the density of the test block, and gypsum whiskers formed connections at the micro-fissures of the test block, preventing the generation and expansion of micro-cracks, improving the internal pore structure of the test block, and enhancing the comprehensive performance of phosphogypsum-based foamed building gypsum.