Monodispersed SiO₂ particles functionalized with cationic polymers poly-((3-acrylamidopropyl)trimethylammonium chloride) (PAPTCl) were prepared using mussel inspired surface modification strategy and surface initiated atom transfer radical polymerization (SI-ATRP). Fourier transform infrared spectroscopy, transmission electron microscope, thermogravimetric analysis, X-ray photoelectron spectroscopy, and zeta potential were employed to characterize these SiO₂ samples. The adsorption performance of the functionalized SiO₂ (donated as SiO₂-PDA-PAPTCl) towards anionic organic dye Congo red (CR) was investigated to evaluate their potential environmental applications. We demonstrated that the surface of SiO₂ particles can be successfully functionalized with cationic PAPTCl. The adsorption capability of as-prepared SiO₂ was found to increases from 28.70 and 106.65 mg/g after surface grafted with cationic polymers. The significant enhancement in the adsorption capability of SiO₂-PDA-PAPTCl is mainly attributed to the introduction of cationic polymers. More importantly, this strategy is expected to be promising for fabrication of many other functional polymer nanocomposites for environmental applications due to the universality of mussel inspired chemistry and well designability and good monomer adaptability of SI-ATRP.