Abstract Graphene oxide (GO) sheets are introduced to stabilize the shape of the most widely studied phase change material for thermal storage, polyethylene glycol (PEG) during the solid–liquid phase change process. In this composite, a maximum weight percentage of PEG as high as 96% without any leakage till a temperature as high as 150°C, far above the melting temperature of PEG, has been achieved. Such a weight percentage of PEG is the highest value among PEG based shape-stabilized PCMs to the best of our knowledge. The shape-stable PCM presents a high heat storage capacity of 142.8J g−1 and excellent thermal reliability within at least 200 melting/freezing cycles. The thermal properties of the PEG/GO composite PCMs with various GO contents are also investigated. The present study provides a highly potential shape-stabilized composite PCM with a high content of PEG for thermal energy storage as well as a way to enhance the heat storage density of PEG based shape-stabilized PCM.