Abstract: Sepiolite (ST) was used as a supporting matrix in compiste phase change materials (PCMs) due to its unique microstructure, good thermal stability, and other raw material advantages. In this paper, microwave acid treatment were innovatively used for the modification of sepiolite. The modified sepiolite (ST_m) obtained in different hydrochloric acid concentrations (0.25, 0.5, 0.75, and 1.0 mol·L⁻¹) was added to stearic acid (SA) via vacuum impregnation method. The thermophysical properties of the composites were changed by varying the hydrochloric acid concentration. The SA-ST_m0.5 obtained by microwave acid treatment at 0.5 mol·L⁻¹ hydrochloric acid concentration showed a higher loading capacity (82.63%) than other composites according to the differential scanning calorimeter (DSC) analysis. The melting and freezing enthalpies of SA-ST_m0.5 were of 152.30 and 148.90 J·g⁻¹, respectively. The thermal conductivity of SA-ST_m0.5 was as high as 1.52 times that of pure SA. In addition, the crystal structure, surface morphology, and microporous structure of ST_m were studied, and the mechanism of SA-ST_m0.5 performance enhancement was further revealed by Brunauere Emmett Teller (BET) analysis. Leakage experiment showed that SA-ST_m0.5 had a good morphological stability. These results demostrate that SA-ST_m0.5 has a potential application in thermal energy storage.