Structures of CaSO4 whisker(A) and CaSO4 whisker(B) were characterized by IR and XRD, and the morphologies were observed by optical microscope. The results showed that the modified CaSO4 whisker(B) has perfect crystal structure, high crystallinity and less defects. Mechanical properties and static thermal stability time(190 °C) of two composites which were prepared by CaSO4 whisker(A) / PVC and CaSO4 whisker(B) / PVC were tested separately, and the degradation processes were analyzed by TG at 50 °C-400°C, N2 atmosphere, and the dispersing states of two whiskers in the composites were observed by means of optical microscope and SEM. The research results showed that the tensile strength and elongation at break increased over 3 MPa and 40% separately, and the static thermal stability time was over 90 min of CaSO4 whisker(B) / PVC, the initial degradation time was delayed comparing with CaCO3 / PVC composite. The consistency and interfacial configuration between whisker and PVC resin were perfect, and the toughness and thermal stability of the composite was improved after CaSO4 whisker was modified. The research results indicated that modified CaSO4 whisker has better toughness and thermal stability actions for PVC.