Abstract: The incorporation of microencapsulated phase change materials (MPCM) into cement-based composite materials can impart latent thermal storage capacity and temperature regulation functionality, demonstrating significant potential for applications in building energy conservation. However, the incorporation of MPCM generally leads to a pronounced deterioration in the mechanical performance and thermal conductivity of cement-based materials, thereby limiting their practical application. The co-incorporation of graphene nanoplatelets (GNPs) and carbon fibers (CF) into phase change heat storage mortar is expected to enhance both the thermal transfer efficiency and mechanical properties of the composite. In this study, the contents of MPCM, GNPs, and CF were selected as the primary experimental variables. Based on mechanical performance tests, scanning electron microscopy (SEM), pore structure analysis, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), thermal conductivity measurements, and small-scale room temperature regulation tests, the effects of GNPs and CF contents on the thermophysical properties, mechanical performance, and thermal stability of MPCM-based phase change energy storage mortar were systematically investigated. The experimental results indicate that increasing the MPCM content leads to a continuous reduction in both the mechanical performance and thermal conductivity of phase change cement mortar. However, the co-incorporation of GNPs and CF enhances the mechanical performance while simultaneously improving the thermal conductivity. TGA results demonstrate that the addition of GNPs and CF has no significant effect on the thermal stability of the phase change cement mortar. The results of the small-scale room experiment indicate that the incorporation of he incorporation of GNPs and CF can enhance the heat transfer efficiency of phase change thermal energy storage mortar while maintaining indoor temperature stability. Furthermore, the optimal contents of MPCM, GNPs, and CF were determined to be 20%, 0.06%, and 1% by mass of cement, respectively.