The invention particularly relates to a preparation method for a silicon-dioxide-based carbon fiber composite adsorbent. Firstly, tetraethyl orthosilicate (TEOS), redistilled water, absolute ethyl alcohol, N,N-dimethylformamide and ammonia water are mixed to prepare sol, then nanometer active carbon fiber is added, an extremely slow heating mode is adopted to heat a mixture to a certain temperature, a gel mixture is obtained and then subjected to aging, solvent exchange and hydrophobic modification, gel is cleaned by n-hexane, drying treatment is performed finally, and the silicon-dioxide-based carbon fiber composite adsorbent is obtained.