Building energy consumption is a major global concern, making energy-saving solutions essential. In this study, phase change thermal storage floors (PCMs) offer an effective solution by efficiently storing and releasing heat. Integrating PCMs into radiant heating systems enables heat energy migration through latent heat, enhancing temperature regulation and reducing fluctuations caused by thermal inertia. This study developed a new phase change heat storage floor using a hydrated salt composite stereotyped phase change material (EP-SPCM) with a phase change temperature of 41.2 °C and latent heat of 170.5 J/g. A laboratory model of underfloor heating was constructed, and the coupled phase change thermal floor-room air heat transfer process was simulated and analyzed by FLUENT software. The results show that the thermal inertia effect of the phase change thermal storage floor can significantly suppress the indoor temperature fluctuation, and at the same time effectively prolong the thermal comfort time through the latent heat regulation of the phase change process, so as to improve the stability of the indoor thermal environment.