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Carbon fibers have excellent properties; however, their wide application is limited by the high cost. In the production process of carbon fibers, the stabilization is the most time-consuming step. If the heat emission can be reduced or its distribution widened, it will be helpful to improve the speed of the stabilization process. In the previous article, we have reported the effect of temperature on the structure of polyacrylonitrile (PAN) in ionic liquids. In this work, influence of residence time on the structures of PAN in ionic liquids is studied by a twin-screw extruder. The formation of C=C and C=N groups was indicated by Fourier transform infrared spectroscopy (FTIR) and Ultraviolet Visible (UV–vis) absorption spectra, which was attributed to the occurrence of cyclization and dehydrogenation during the extruding process. The cyclization degree was calculated by differential scanning calorimetry (DSC) test, which can be up to 29.4% with the residence time of 40 min at 210 °C. Therefore, this method can be helpful to improve the production efficiency in the subsequent stabilization process by widening and reducing the distribution of heat emission in the product process of PAN fibers.