An annular optical element includes a first side surface, a second side surface, an outer annular surface and an inner annular surface. The outer annular surface connects the first side surface and the second side surface, and surrounds a central axis of the annular optical element. The inner annular surface connects the first side surface and the second side surface, surrounds the central axis, and is closer to the central axis than the outer annular surface is to the central axis. The inner annular surface includes a plurality of protrusion circles surrounding the central axis and arranged along a direction from the first side surface towards the second side surface. Each of the protrusion circles includes a plurality of protrusion structures and a plurality of separation structures, and the protrusion structures and the separation structures are alternately arranged to surround the central axis.