The theoretical and finite element models were established for the gas-assisted extrusion from square die.The three dimensional non-isothermal computer simulation were carried out with the use of Fidap soft ware.The analysis and comparson showed that compared with the change of shape and size in conventional extrusion,gas-assisted extrusion could reduce the die swell effectively.So the shape and dimension precision of the extrudate could be improved which was helpful for the polymer extrusion processing with high speed and high quality.