The mechanism of host--guest interaction driven phase change behavior is still unclear, although many pore-based composite phase change materials (PCMs) have been synthesized and tested. Here, we prepared stearic acid (SA)-metal organic framework MIL-101(Cr) composite PCMs. FTIR, PXRD, DSC, and 3ω measurements were used to evaluate the constructional and phase change thermal properties of the obtained composites. With a pseudo-supercritical path (PSCP)-based molecular dynamics method, the melting temperature and enthalpy in a complex restricted space were well predicted, and the radius of gyration, interaction energy, atom-atom radial distribution functions were further analyzed. The results showed that the surface modification highly improved the loading capacity for SA, from 30 wt% to 70 wt%, with a fusion enthalpy increasing from 46.3 J/g to 110.01 J/g, owing to the new formation of hydrogen bonding between modified surface and PCM. Thermal conductivity of the host--guest PCM was increased by 68.2%, which was higher than that of either the MIL-101(Cr)NH2 or the SA alone. It was important to study the underlying mechanism of modified surface on energy storage in confined nanopores. The discussion suggested a controllable design and preparation for high performance PCM composite. Display omitted •Amino groups were successfully grated onto MILs by surface modification.•The PSCP method was first applied for well-prediction of complex composites melting point.•There was a great improvement on loading capacity, due to the newly formed hydrogen bonding.