An optical system comprises a light source and a light integration module, which comprises a light integration rod, an optical assembly and an anti-reflection coating. The light integration rod has an entrance, which is covered by the optical assembly. The optical assembly has a transparent surface, and a surface area of the transparent surface is greater than a cross-sectional area of the entrance of the light integration rod. The anti-reflection coating is formed on the transparent surface. After an incident light is transmitted through the anti-reflection coating, it is transmitted through the optical assembly along a light path, and then outputted to the entrance of the light integration rod.