Based on the Warren's interface theory of liquid-solid interfacial energy for complicated compatible binary and pseudo binary systems, a physical model for the liquid-solid free interfacial energy that has been simplified at the theoretical level was obtained by taking the Pb-Al liquid-solid binary system as an example. A theoretical formula with two variables was then obtained by deducing a thermodynamic equation, and the calculational values and the experimental values by multiphase equilibrium method (MPE) were compared. Finally, research on a Pb-Al binary liquid-solid system showed that the calculated liquid-solid interfacial free energy (γSL) values from the model provided a result similar to the experimental measurement with a low average error. The calculated values only depend on the temperature and the Al concentration. A foundation to calculate γSL for other systems has therefore been established.