Mushy zone is where the melting or solidification actually begins and proceeds, accompanying which the latent heat absorbs or releases. Thus, it is critical for the realization of latent heat storage. In this paper, attempts were made to examine the melting and solidification processes from a new perspective, that is, the mushy zone evolution, which tightly couples with the heat transfer, fluid flow, and phase change. A conjugate heat transfer model was developed including the lauric acid in a basic rectangular latent heat storage unit, the surrounded thermal insulations, and the ambient air. The enthalpy-porosity model was used and the mushy zone constant was reasonably evaluated to accurately predict the solid-liquid phase change processes. The model was validated against the experimental data from literature. In addition, a detailed description on the coupling of heat transfer, fluid flow, and phase change within the mushy zone was presented. It has been found that the mushy zone was highly suppressed during melting while widely extended during solidification. Moreover, the effects of the isothermal assumption for non-isothermal phase transition as well as mushy zone constant on the predicted heat storage performance were investigated. The results obtained can give some insights into the enthalpy-porosity method to accurately predict the mushy zone phase change processes. Meanwhile, more clues can be achieved to design more efficient heat storage devices and to develop better phase change materials.