The invention discloses ultralow-density fluffy silica fiber and a preparation method thereof. According to the preparation method, tetraethyl orthosilicate as a silicon source and polyvinyl alcohol as a polymer carrier are subjected to crosslinking, so that a tetraethyl orthosilicate/polyethylene solution has specific properties; collected fiber is self-assembled and stacked at the room temperature environment by an ordinary electrostatic spinning device, a three-dimensional fluffy structure is formed, the fiber is calcified in a muffle furnace at the high temperature, and the fluffy silica fiber is obtained. The prepared fluffy silica fiber has the diameter of 0.4-2 mu m, is resistant to high temperature, has the apparent density being 0.016 g/cm<3>, can absorb water with the weight being 75 times that of the fluffy silica fiber, and can still keep the original fluffy three-dimensional structure after being repeatedly used. The preparation method is simple, economical, practical, low in cost and easy to popularize and does not require special materials or devices.