Both aminopropylisobutyl polyhedral oligomeric silsesquioxane (NH₂-POSS) and hydrazine monohydrate were utilized to functionalize graphene oxide (GO), and to obtain chemically modified graphene (CMG), which was then used for preparing thermally conductive CMG/polyimide (CMG/PI) nanocomposites via a sequential in situ polymerization and electrospinning-hot press technology. NH₂-POSS molecules were grafted on the GO surface, and CMG was obtained by the reaction between NH₂-POSS and GO. The thermal conductivity coefficient (λ), glass transition temperature (T_g) and heat resistance index (T_HRI) of the prepared CMG/PI nanocomposites were all increased with increasing the CMG loading. The λ value of the CMG/PI nanocomposites with 5 wt% CMG was significantly improved to 1.05 W m⁻¹ K⁻¹, about 4 times higher than that of the pristine PI matrix (0.28 W m⁻¹ K⁻¹). The corresponding T_g and T_HRI values were also increased to 213.0 and 282.3 °C, respectively. Moreover, an improved thermal conductivity model was proposed and predicted the λ values of the nanocomposites more precisely than those obtained from the typical Maxwell, Russell and Bruggemen classical models.