The invention discloses a fixed length control system. The fixed length control system comprises a rack, an unreeling device, traction devices, swing arm type tension adjusting devices, a coating device, a drying device and a take-up device. When the fixed length control system is in a debugging stage, a numerical value is set for the tension of an unreeled film between a traction mechanism and a coating mechanism, electric eyes are used for detecting the state of the film which is subjected to the tension, and if a pattern does not reach a standard length value, the tension of the film segment is improved through an operation interface; if the pattern length exceeds the standard value, the tension of the film segment is reduced through the operation interface till the pattern length is adjusted to the standard value; the tension of a film between a coating mechanism and a traction cooling mechanism is adjusted in a manner of the tension adjusting of the film segment; the length of the film is shaped when the film is in the drying device; and by means of cooling of the traction cooling mechanism, the situation that the film cannot be opened after being rolled up due to chilling shrink is prevented.