The effects of styrene-co-acrylonitrile resin (AS) on the mechanical properties, morphology, and plasticizing and theological behaviors of poly(vinyl chloride)/chlorinated polyethylene (PVC/CPE) blends are studied. The results show that the impact strength and the tensile strength are all increased and the plasticizing and theological behaviors are also improved when a certain amount of AS is added into PVC/CPE blends, which are different in characteristics and regularity from plastics toughened with elastomers. It is blends of brittle-ductile transition regions (i.e., PVC/CPE = 100/10, 100/15) that can obviously be toughened by AS. The analysis of the morphological structure shows that AS promotes the formation of a CPE network that embeds the primary particles of PVC.