Developing novel shape-stabilized phase change materials (ssPCMs) is of vital importance in utilization of thermal energy. In addition, the reveal of interaction mechanism between PCM and supporter, is a prerequisite for design and synthesis of ssPCMs. In this study, PEG was filled into mesoporous silica MCM-41 via the solution impregnation method to prepare composite PCMs. The -NH2 groups were successfully grafted onto the surface of MCM-41, by replacing original -OH groups. It turns out that the phase transition of PEG did not occur in hydroxyl-modified MCM-41 even at the maximum loading of 70 wt%. Excitingly, after amino modifying, the composite PCMs had superior properties compared with PEG/MCM-41-OH. The phase change was available even at 30 wt% loading and the phase change enthalpy was increased from 0 J/g to 58.76 J/g. Furthermore, the ssPCM exhibited good thermal reliability after the thermal cycling. Mechanism within the experimental phenomena was studied by molecular dynamics method. Excessive adsorption occurs at the nanochannel surface under the strong interaction between -OH groups and PEG molecules thus restrains the phase transition of PEG. Amino groups reduced the host-guest hydrogen bonding interaction, which is conducive to PEG crystallizing. It is concluded that the PEG/MCM-41-NH2 is a promising candidate for use as low temperature solar energy storing due to the improved properties and simple operation. Moreover, MD results suggested that the host-guest interaction could be regulated by surface modifying therefore to control the loading and crystallization behavior of PEG in MCM-41, and the methodology can be extended to other nanoporous-based PCMs. Display omitted •Low temperature composite PCM PEG/MCM-41was fabricated.•The PEG/MCM-41-NH2 showed superior phase change performance than PEG/MCM-41-OH.•The underlying mechanism was investigated by MD simulation, and host-guest polarity matching was proposed for ssPCM design.