Abstract: The modified diatomite was used as the main filler, polypropylene(PP) resin as the matrix, epoxidized soybean oil (ESO) as the plasticizer and heat stabilizer, the modified diatomite-ESO/PP composites were prepared by melting blending method. The factors of influence and strengthen toughening mechanism was analyzed by XRD, SEM, optical microscope (OM), polarizing microscope (PLM) and mechanical properties test. The results show the space grid structure of the modified diatomite-ESO/PP can be formed through the combination of physical crosslinking or chemical grafting among the interface of diatomite, ESO and PP, and diatomite can disperse in PP matrix uniformly. When modified diatomite mass ratio remains invariability, with the increase of ESO content, the flexural strength of the modified diatomite-ESO/PP decreases slightly, but the impact strength and hardness increase. With 20% mass ratio to PP of modified diatomite and 2.5% mass ratio to PP of ESO, the modified diatomite-ESO/PP composite with optimal mechanical properties can be obtained. The reinforcement of the modified diatomite-ESO/PP is mainly beacause that the modified diatomite has some characteristics such as a high modulus, the heterodromous nucleation in the polymer melt promoting the crystal of PP, a better interfacial adhesion with matrix, and a good dispersibility. ESO can insert into polymer molecular chains, weak the mobility among the polymer molecular chains, and to a certain extent reduce the crystallization of the matrix, which lead to improve the toughness of PP matrix. Due to the modified diatomite adding to the PP matrix, α crystal of PP transferms into β crystal of PP with higher impact strength, thus the modified diatomite and ESO can work cooperatively and achieve reinforcing and toughening of the modified diatomite-ESO/PP composite. However, more adding of ESO makes the modified diatomite focus on the surface of composites, and results in the increase of the surface hardness of the modified diatomite-ESO/PP composite.