An annular optical element includes an outer annular surface, an inner annular surface, a first side surface, a second side surface and a plurality of strip-shaped wedge structures. The outer annular surface surrounds a central axis of the annular optical element and includes at least two shrunk portions. The first side surface connects the outer annular surface and the inner annular surface. The second side surface connects the outer annular surface and the inner annular surface, wherein the second side surface is disposed correspondingly to the first side surface. The strip-shaped wedge structures are disposed on the inner annular surface, wherein each of the strip-shaped wedge structures is disposed along a direction from the first side surface towards the second side surface and includes an acute end and a tapered portion connecting the inner annular surface and the acute end.