A fuzzy control strategy on viscosity of a polymeric melt during the extruding processes is proposed to ensure producing the quality-consistent end products. Because the viscosity model in the control system varies with respect to different conditions of the system command (e.g., screw speed), we focus on using a fuzzy controller accomplished with a fuzzy modeling technique to solve the model-variant viscosity control system. Also, system stability analysis is presented. According to the simulation and the implementation results, better performances and a suitable using method are concluded.