The confinement effect of the solid liquid phase in porous materials was explained only qualitatively in previous research because the pore structure is complicated and difficult to describe quantitatively. In this work, fractal theory was adopted to characterize quantitatively the pore shape by surface fractal dimension. A quantitative model that disclosed the relationship among the solid liquid phase-change enthalpy, pore size, pore shape, and interfacial groups was proposed for phase change materials confined in a porous matrix. Furthermore, the model was applied to hydrated salts/silica composite and hydrated salts/expanded graphite composite; the results show that the fitted values agree well with the experimental values.