For polylactide (PLA) dedicated screw rod of different type of melt conveying paragraph, using Polyflow software for three dimensional flow field simulation three species different type mixed refining structure, analysis polylactide extrusion process of three dimensional isothermal flow field. Results showed that: in same of process conditions, different of single screw rod structure conditions, the physical properties of flow road within polylactide exists differences; ordinary type single screw rod of melt body of axis to speed and pressure larger, conducive to screw rod extrusion process, but melt body shear rate is smaller, and melt viscosity is bigger, led to extrusion melt body of quality poor; the axial velocity and shear rate of pineapple type single-screw melts is larger, but viscosity is smaller, so it has better plasticizing ability; pineapple type single-screw and pin type single-screw in circulation will flow in the case, and is not conducive to melt-conveying.