The biocomposites of poly (lactic acid) (PLA) involving 15% mass fraction of calcium sulfate whiskers (CSW) were prepared via melt-blending technology, in an effort of toughening PLA and enhancing the thermal stability. The morphological structure, impact toughness, thermal stability as well as kinetic analysis on thermal decomposition for PLA/CSW composites were performed thoroughly. The results showed that CSW was organized successfully via silanization, helping to form well-bonded interfaces, and accordingly, the impact toughness increased remarkably. The thermal stability was enhanced by adding whiskers, leading to increased decomposition temperature and decreased mass conversion rate. Kinetic analysis revealed the great dispersions on the reaction order and activation energy. Though, in comparison to pure PLA, the reaction order of PLA/CSW composites increased based on calculation methods except for Carrasco's, the activation energy of the composites declined independently of the applied mathematical models, meaning that thermal decomposition of PLA phase was accelerated by the introduced CSW.