Objective: Taking polylactic acid （PLA） as an example, the flow channel distribution of twin-screw extrusion under different pitch was studied. Methods: In Solidworks, two 3D models with variable pitch and constant pitch were drawn according to the theoretical end curve equation and modified. Workbench software was used to generate the required analysis models with the swept mesh method for the two screw configurations, and then Polyflow fluid simulation software was used to simulate the actual situation. After obtaining the post-processing results, FieldView software was used to analyze the final results. The pressure, shear rate and viscosity of polylactic acid fluid under simulated conditions were studied. Meanwhile, the residence time distribution curve RTD was analyzed by Polystat statistical module. Results: Under the same conditions, the pressure, shear rate and viscosity fluctuation of the screw with variable pitch increased, and the axial mixing performance improved, but the self-cleaning ability decreased. Conclusion: The change of pitch has a certain effect on polylactic acid, but too large change of pitch often produces adverse effects. Therefore, it is necessary to design suitable variable pitch screw according to the actual situation to improve the yield and quality.