An innovative way to enhance the thermal conductivity of composite phase change materials (PCMs) is by carbonizing the carbohydrate-coated mineral substrate to modify its porous structure. Here, we study the effect of introducing different amounts of carbonized starch acting as a filler for enhancing the thermal conductivity of the shape-stabilized composite PCMs (ss-CPCMs) based on polyethylene glycol (PEG) and modified expanded vermiculite (MEV). The experimental exploration of the thermal behavior of composite PCMs has been explored sufficiently in depth. The MEV is shown by SEM images to tightly adhere to the carbon layer and to be composed of a dense micropore structure. Results from DSC indicate that the latent heat of the PEG/MEV ss-CPCMs gradually decreases with the increase of carbon layer mass fraction. Results obtained from FT-IR and TGA show that PEG/MEV ss-CPCMs exhibit good chemical and thermal stabilities. The thermal conductivity of the PEG/MEV ss-CPCMs increases with the carbon layer mass fractions and reaches up to 1.94 times that of PEG/EV ss-CPCMs. The PEG/MEV ss-CPCMs result in a promising material for applications in energy-efficient buildings due to its excellent thermal properties, ideal thermal conductivity and good chemical and thermal stability.