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The leakage of Na2HPO4•12H2O phase-change materials (PCMs) in the liquid state can be prevented by encapsulating them into porous expanded vermiculite (EV) to prepare composite PCMs. The main issues of EV-based composite PCMs are the low thermal conductivity and the crystal water loss of inorganic hydrated salt. Graphene oxide (GO) can help enhance the thermal conductivity of composite PCMs and firmly adsorb water molecules through oxygen groups. The composite orders of GO play an important role in adjusting the thermal properties of composite PCMs because the suitable positions of GO in the stratified structure of EVs would positively influence the PCMs. The Na2HPO4•12H2O/EV composite with GO was prepared in three different orders. Compared with the common method that involves compositing GO in Na2HPO4•12H2O, compositing GO on the hole wall of the EV exhibited a higher thermal conductivity of 0.54 W/m•K and compositing GO out of Na2HPO4•12H2O/EV exhibited better thermal stability, which incurred a weight loss of 4.14% at a high temperature. A thermal energy storage behaviour analysis indicated that GO-modified PCMs show a large heat storage capacity (melting process: 98–126 J/g; solidification process: 56–91 J/g). Two new composite orders positively affected the heat storage of the composite PCMs.