<p>The temperature rise caused by shear action during melting for high molecular polymer is widespread and difficult to control, especially for the co-rotating twin-screw extrusion (CRTSE) process. Considering the very small gap between the screw and the barrel, it is difficult to in-line measure and control the value of materials’ temperature along the extrusion direction. Consequently, a co-rotating twin-rotor mixing apparatus with a high-precision temperature sensor is employed to explore the temperature variation of polyethylene (PE) with different average molecular weights (M_w) from solid to melt, similar with the CRTSE process, but the material will not be conveyed forward. The experiment’s result indicates that a higher rotor speed and a lower meshing clearance result in a higher temperature rise. Meanwhile, unexpectedly, an interesting experimental phenomenon is observed: the temperature rise caused by shear action in high-viscosity PE increases with the rotor speed, while it enlarges initially and then stabilizes in low-viscosity PE as the increased rotor speed. Therefore, it is speculated that the temperature rise during the processing strongly depends on the viscosity. Simulation studies based on Ludovic software by actively varying the consistency index and power-law exponent confirm the correctness of the experimental conjecture under a wider screw speed and viscosity range. This study has theoretical and practical implications for a deeper understanding of the temperature rise caused by shear action in polymeric materials during extrusion.</p>