The present invention relates to a filter, a CMY color mixing assembly using the filter and an optical system thereof. Said filter comprises a substrate and a filtering film plated on the substrate, and the filtering film comprises a uniform region and a color mixing region. The thickness of the filtering film in the uniform region is uniform, while the thickness of the filtering film in the color mixing region is not uniform. The thickness of the filtering film in the uniform region is greater than or equal to the thickness of the filtering film in the color mixing region, and the thickness of the filtering film of a central region of the color mixing region is greater than the thickness of the filtering film of side regions of the color mixing region. Said color mixing region is provided at a side where the filter first accesses to a light path.