A series of alpha-methylstyrene, styrene and acrylonitrile(alpha-MSAN) copolymers with different alpha-methylstyrene (alpha-MSt) content were synthesized by altering alpha-MSt and St ratios with emulsion copolymerization method. By melt blending these copolymers with PVC resin and di-isooctyl phthalate (DOP), PVC/alpha-MSAN and PVC/alpha-MSAN/DOP blends were prepared. The miscibility and morphology of the blends were investigated by dynamic mechanical analysis (DMA) and scanning electron microscopy (SEM). The miscibility of PVC/alpha-MSAN blends is substantially improved with the increasing alpha-MSt content in alpha-MSAN copolymer containing identical AN content and the blends show homogeneous morphology as the alpha-MSt content in alpha-MSAN copolymer is up to 22.5wt%. alpha-MSAN copolymer containing 37.5wt% alpha-MSt is fully miscible with PVC throughout the whole composition range. When DOP was introduced into the PVC/alpha-MSAN blends, a single Tan delta peak over room temperature in DMA detection is found as alpha-MSt content in alpha-MSAN copolymer is from 15 to 75 wt%, and morphology result also shows that the PVC and alpha-MSAN copolymer is miscible under the influence of DOP.