A folded optical system includes an optical folding assembly and lens elements. The optical folding assembly has a first transmissive surface, a first reflective surface, a second reflective surface substantially the same interface as the first transmissive surface, a third reflective surface and a second transmissive surface sequentially along an optical path. The optical path reaches the first reflective surface via the first transmissive surface along a first optical axis to be sequentially redirected to a second optical axis, a third optical axis, and a fourth optical axis respectively by the first reflective surface, the second reflective surface, and the third reflective surface and reaches an image surface via the second transmissive surface. The lens elements at least include a first lens element located on the first optical axis. The first lens element with positive refractive power has a light incident surface being convex in a paraxial region thereof.