Abstract: This Paper introduces a self-adaptive tension control system used for the unwinder in a polyester film slitter. Siemens S5-115UPLC is used in the system and 6SE7024 vector control converter is used as its AC driver. Several measured signals are transmitted to PLC via ET-200 distributed I/O module and Profibus-DP. In this paper the tension control principle of the unwinder is introduced. Because the range of the roller diameter varies widely, its moment of inertia also varies wildly. For this reason the principle of self adaptive control is used in the system. The SR loop of the system is made up of PLC, Converter and photoelectricity encoder. In the control channel of the SR loop a parameter is chosen, which is proportional to the square of the roller diameter, to compensate for the variation of the moment of inertia. By this way the SR loop can possess good dynamic properties the whole time during the variation of the roller diameter. The simulation results prove the validity of the method and the good tension control effect is gained in application.