Adding nano-particles with high refractive index can greatly improve the optical properties of the PMMA-based composites, which were developed as the new-type rear projection screen. In this paper, the influence on the optical properties, such as gain and view angle, by the content of additive TiO2 nano-particle was studied. Curves of optical properties versus content of nano-particle were obtained and the rational factors were found. Finally the reasons were discussed from the nanostructure point of view and the scattering principle.