Compared with traditional materials, the phase change materials based on latent heat have higher heat storage efficiency. In order to study the heat storage mechanism of phase change materials in a square cavity, the momentum satisfying the phase change was modified based on the Boussinesq hypothesis, and the fluid-solid-thermal coupling calculation model for melting heat storage of phase change materials heated from below was established. Selecting the organic phase change paraffin, the melting experiment of paraffin under bottom constant temperature was carried out to verify the correctness of the calculation model. The results show that the melting process is dominated by conduction and natural convection heat transfer. At the initial stage of melting, the melting front is basically parallel to the heating surface, and the heat transfer is mainly provided by conduction. When the liquid thickness exceeds 2mm, the natural convection heat transfer is gradually activated to accelerate the melting rate, and forms the irregular melting front. According to the characteristics of melting front and liquid flow, the whole melting process can be divided into four stages: conductive regime, stable growth regime, transition regime and turbulent regime. In addition, the natural convective heat transfer in liquid phase has a significant size effect on the improvement of melting heat storage efficiency of phase change paraffin, and intensified with the increase of cavity size. When the square cavity size is less than 2mm, the improvement efficiency of natural convection heat transfer is less than 1% and can be ignored.