The invention relates to an automatic control system for monitoring the film thickness of optical filming. A light projecting device is directly connected with a light entrance port of a film coating machine or is connected with an optical splitter and a reflector chest for projecting light generated by the light projecting device into the film coating machine, a light exit portal of the film coating machine is connected with a light transporting element which is connected with a light entrance port of the optical splitter, a light exit portal of the optical splitter is further connected witha light entrance port of a photoelectric signal changeover gear, or an exit ray of the film coating machine reflects light into the photoelectric signal changeover gear through the reflector chest, an electrical signal output end of the photoelectric signal changeover gear is connected with a computer through a signal processing unit. The invention further relates to an automatic control method for monitoring the film thickness. The invention improves the stability and accuracy of monitoring the film thickness and meets the requirement of filming with high degree of accuracy.