Differences in the synthesis methods can affect the performance of nanocomposite materials. The synthesis methods of SiO2-based nanocomposite pour point depressants synthesized by chemical hybrid are mostly in situ polymerization and chemical grafting. However, there are still some gaps in the synthesis of nanocomposites using chemical grafting. In this paper, the amino-modified Nano-SiO2 and octamethyl methacrylate-maleic anhydride copolymer was used to prepare PM18-g-NSiO2 by an amidation reaction, and the product was compared with PM18/SiO2 prepared by the solution blending method. The characterization results of FT-IR and SEM showed that the chemical bond formed between PM18 and NSiO2 and PM18-g-NSiO2 showed a more regular morphology structure. The results of rheological tests showed that the optimal concentration of PM18-g-NSiO2 and PM18/SiO2 is 300 mg·kg−1, but the decrease of PM18-g-NSiO2 is 19 °C, while the decrease of PM18/SiO2 is only 13 °C. The lowest viscosity of model oil added PM18-g-NSiO2 was only 214 mPa·s while PM18/SiO2 reached 453 mPa·s. The wax crystal structure after adding PM18-g-NSiO2 is also more regular and smaller compared to PM18/SiO2. We concluded that differences in the synthesis methods may lead to the different coverage of the polymer on the surface of the nanoparticle.