Objective: Explore the effects of different combinations of forward screw elements in the melt conveying section on the melt conveying characteristics of reverse screw elements. Methods: Two different models were constructed by changing the lead, using SolidWorks software to establish a three-dimensional model, and using Polyflow software to numerically simulate the extrusion process of the two models in the melt conveying section. The effects of the screw groove width and screw edge thickness of the screw elements on the flow field under different lead lengths and the changes in the flow field at different rotational speeds were analyzed, and the effects of rotational speed on the material mixing were verified through experiments. Results: The constructed model can allow the melt to pass through the reverse threaded element. At the same screw speed, the pressure building capacity and conveying capacity of model one constructed from large lead screw elements were stronger than that of model two constructed from small lead screw elements. Model one was superior to Model two in terms of decentralized mixing ability, and weaker than Model two in terms of distributed mixing ability. Under the same pressure difference at both ends, increasing the rotational speed could improve the dispersion and mixing effects of materials. Conclusion: Using model one in the improved processing of polylactic acid and increasing the rotational speed of the screw can further crush the material and improve the quality of the extrusion.