Because phase change materials (PCMs) have their own phase change temperature ranges, the real phase change processes are not isothermal. PCMs may be in partially-melted state during the practical application. In this paper, the phase change performances of sodium acetate trihydrate (SAT) in 3 kinds of states, which are no-melted, partially-melted and fully-melted respectively, are researched by experiments. It is found that the state of SAT affects its phase change performance. When SAT is in partially-melted state and the melting ratio is low, SAT can release latent heat during cooling process and the degree of supercooling is relatively small. When the melting ratio of SAT is high or SAT is in fully-melted state, it has large supercooling degree and can not release latent heat. The results also show that the supercooling degree of SAT increases with the increase of the heating time when it is in partially-melted state.