The work aims to study the effects of the type, particle size and amount of inorganic filler on the mechanical properties and thermal stability of PF/HDPE composites. PF/HDPE composites were prepared by melt blending and injection molding with poplar fiber (PF), high-density polyethylene (HDPE), BaSO4, CaCO3, and mica powder as raw materials to conduct mechanical analysis, thermogravimetric analysis and SEM analysis. The results showed that all the three inorganic fillers improved the mechanical and thermal stability of the PF/HDPE composites, and the properties of composites filled with CaCO3 were better than those filled with BaSO4 and mica powder. The improvement effect was enhanced with the decrease of the filler particle size. The amount of filler needs to be kept within a certain range. Too low or too high would cause a decline in performance. The PF/HDPE composites with CaCO3 (mass fraction 9%, 3000 mesh) has the best mechanical and thermal stability.