Microstructure, reacting degree and main properties of synergistically modified unsaturated polyester resin (UPR) with polyester urethane (PU) and organically-modified montmorillonite (OMT) are studied in this paper by scanning electron microscopy (SEM), X-ray diffraction (XRD), FT-IR, contact angle and mechanical properties. The research shows good compatibility of PU with natural fibre and that the contact angle of modified UPR and natural fibre is decreased. Nanocomposites are made using OMT and the impact section of synergistically modified UP composites with PU and OMT shows the toughness breaking. With the increased addition of PU and OMT, the impact strength is increased, the flexural modulus and the cured shrinkage rate are decreased. The testing of mechanical properties shows that when the content of PU is 5% and that of OMT is 1%, the impact strength is increased 80% or above, the decrease of flexural modulus is less than 20% and the cured shrinkage rate is less than 4%. A PU and OMT system was found to have a synergistic effect on the toughness of UPR.