HDPE/CSW composites were prepared by melt blending method using γ-methacryloxypropyltrimethoxysilane (KH570) modified calcium sulphate whiskers (CSW) as filler for high density polyethylene (HDPE). The effects of KH570-modified CSW on the properties of the composite HDPE/CSW were characterized by SEM, XRD, TG and DSC. The results showed that the tensile and flexural strengths of HDPE/CSW increased by 39.51% and 15.9%, respectively, compared with pure HDPE at a CSW mass fraction of 20%, and were prone to heterogeneous crystallization, which enhanced the heat resistance and crystallinity of the HDPE/CSW composites, and the crystallization behaviour showed that the HDPE/CSW composites grew as two-dimensional sheet crystals. The activation energy of thermal degradation reaction of the original HDPE was 244.11 kJ/mol, and the activation energy of thermal degradation reaction of HDPE/CSW composites decreased to 236.99 kJ/mol at 50% mass fraction of CSW, indicating that CSW enhanced the thermal degradation reaction rate of HDPE/CSW composites and extended the use range of the composites.