A new composite phase change material (CPCM) for heat storage was prepared via the vacuum impregnation method. The material was based on sodium acetate trihydrate (SAT) adsorbed in micro-porous expanded vermiculite (EV), to form the micro-scale phase change of SAT in each EV micro cell acting as an independent phase change unit. The adsorptive capacity of SAT in EV is up to 600 wt%, while with 2 wt% borax added as a nucleating agent in the CPCM. The micro-structure of EV occupied by SAT was characterized by scanning electron microscopy (SEM) images. X-ray diffraction (XRD) tests indicated good chemical compatibility among the different components of the CPCM. The phase change temperature and heat storage density of the CPCM were 57.6 degrees C and 270.6 kJ/kg respectively, and the thermal conductivity was higher than that of each component in the compound. Thermogravimetry (TG-DSC) results showed that the insurmountable super-cooling and phase separation problems were almost overcome, and the thermal performance test system results showed that the CPCM was stable over 150 melting-solidification cycles. The performance improvement of SAT is attributed to the micro-scale phase change and heat transfer in EV cells.