The invention discloses a nano silicon dioxide-phenolic resin composite foam material and a preparation method thereof. The composite foam material is prepared from silicon dioxide nanoparticles, free phenol, free formaldehyde, a surfactant, a curing agent and sodium chloride as well as phenolic resin, wherein the silicon dioxide nanoparticles are bonded with phenolic resin by virtue of Si-O-C chemical bonds. The method comprises the following steps: firstly, ultrasonically dispersing a SiO2 sol in a formaldehyde solution or a mixed liquid of formaldehyde solution and paraformaldehyde to obtain a mixed solution; then, after mixing the mixed solution, phenol and an alkaline solution, and carrying out polymerization reaction to obtain resol in which the silicon dioxide nanoparticles are uniformly dispersed; then, firstly, carrying out acid neutralization and reduced pressure distillation on the resol; then, mixing and stirring the resol with a foaming agent and the surfactant to obtain mixed resin; and finally, adding the curing agent into the mixed resin and putting into a die to cure and foam to prepare a targeted product. The nano silicon dioxide-phenolic resin composite foam material disclosed by the invention can be widely applied to the fields such as building, aviation, refrigeration and the like commercially.