The invention discloses a preparing method of a silicon dioxide aerogel nanofiber composite membrane, and relates to nanofiber composite membranes. The method comprises the steps of preparing a silicon dioxide aerogel/organic solvent dispersion liquid; preparing a polymer/SiO2 spinning solution; conducting electrostatic spinning on a polymer/SiO2; conducting post-treatment on the nanofiber composite membrane. Fibers of the prepared silicon dioxide aerogel nanofiber composite membrane are in needle shapes, the average diameter of the fibers is 200-300 nm, and the specific surface area of the silicon dioxide aerogel nanofiber composite membrane can reach 55 m<2>/g; the process is simple, the preparing condition is mild, and the obtained composite nanofiber membrane is large in specific surface area and small in fiber diameter and is expected to be applied to the fields of heat insulation, adsorption, catalysis, bioengineering and the like.