A series of silica/poly (methyl methacrylate) grafting composites (SiO2/PMMA) with different grafting and crosslinking structures were used to prepare poly (methyl methacrylate)/silica (PMMA/SiO2) composite fiber via solution blending and electrospinning processes. The dispersion of silica in PMMA/DMF solution and in PMMA/SiO2 composite fiber, the distribution of silica in electrospun products and the morphological characteristics of electrospun fiber were measured by dynamic light scattering (DLS), scanning and transmission electron microscopy (SEM & TEM), as well as ultrasonic washing and HF etching experiments. It was found that only the simple-grafted silica with grafting percentage of 484.4% is suitable for preparing electrospun fiber. More than 90% of simple-grafted silica particles can be effectively encapsulated in PMMA/SiO2 electrospun composite fiber, and thus the entire cross section of PMMA/SiO2 electrospun fiber was filled by simple-grafted silica at a primary-particles size level. The above results are favorable for achieving the primary-particles-size-level-dispersion of functional inorganic particles in sub-micron level fiber, and thus can be used to guide the production of functional composite fiber.