Liquid crystal display has become the mainstream display technology, in which the light diffusion sheet can haze the light and ensure good display quality. Quantum dot materials have the function of color conversion and improving color quality, so quantum dot light diffusion sheets are the development direction of the next generation of liquid crystal displays. The liquid crystal display technology with quantum dot backlight as the light source has become more mature and is gradually being promoted to the market. The use of polymeric encapsulation method for quantum dot to form quantum dot diffusion sheet can effectively reduce the manufacturing cost and help expand the market share of quantum dot liquid crystal display. In this paper, polystyrene/styrene-acrylonitrile copolymer (PS/SAN) composite films with different contents of styrene-acrylonitrile copolymer (SAN) were prepared by hot pressing and extrusion casting, and the morphology, mechanical, optical and oxygen barrier properties were investigated. The results show that compared with pure (polystyrene) PS film, the oxygen barrier property of the composite film with the SAN mass fraction of 30% prepared by casting is improved by 29%, the tensile strength is improved by 16.6%, the light transmittance is greater than 80% and the haze is at around 40%. Compared with the film prepared by hot pressing, the composite film prepared by casting shows better oxygen barrier property, which is possible to be used to prepare the diffusion layer for PS-based quantum dot diffusion sheet.