A light mask type optical device includes an optical element. The optical element is formed by a light inlet face, a light outlet face opposite to the light inlet face, and a plurality of grating structures of micro meter (μm) scale disposed on the light outlet face. The grating structure includes a first slant face, a second slant face, and a plane surface between the first slant face and the second slant face. In application, a direct downward light emitted by a light source is reflected by the first slant face and the second slant face, to form a circulating light in the grating structure, and that is combined with the direct downward light to output from the light outlet face in achieving reduced glare and increased lumen in application.