Abstract: Using straw fiber/polypropylene (PP) masterbatch and PP as the main raw materials, straw fiber/PP composites with different fiber contents were prepared by twin-screw extruder. The pressure specific volume temperature (PVT) and viscosity curves of the composites with different straw fiber contents were established by testing and analyzing the PVT and viscosity characteristics of the composites. The influences of fiber contents on the PVT and viscosity properties of the composites were discussed. The modified two-domain Tait equation of state and Cross-WLF viscosity equation were used to fit the parameters of PVT curve and viscosity curve respectively. Based on the analysis of fitting results, the modified PVT model and viscosity equation were established considering the influence of straw fiber content. Applied in Moldflow analysis software, the material properties data file was established, and the mold flow analyses of an automobile door trim panel were carried out. The results show that the established composite rheological equation is effective in numerical simulation analysis of injection molding.