A deposition system and film thickness monitoring device thereof. The film thickness monitoring device for monitoring thickness of a thin film coated on an optical substrate includes a laser light source, a retro-reflector, and a light receiver. The laser light source and the retro-reflector are disposed on opposite sides of the optical substrate. First, a light beam is emitted by the laser light source and then passes through the thin film along a first path. Second, the light beam is reflected by the retro-reflector and then passes through the thin film again along a second path parallel to the first path. Third, the light beam is received by the light receiver.