The invention provides a method for preparing nanofibers, which comprises the following steps that: a spinning solution is sprayed to form charged liquid drops by electrostatic spraying; and the charged liquid drops enter a low-temperature environment and quickly exchange energy with an ambient environment, so that the liquid drops are in an unbalanced state, the deformation of the charged liquid drops is promoted, and fluid is induced from the deformed liquid drops under the action of electrostatic repulsive force of charges and is condensed and solidified to form the nanofibers. The invention also provides a device for preparing the nanofibers by the method. In the method, the fluid is induced from the charged liquid drops and then condensed and solidified, so the method is also suitable for low-viscosity liquid, and the types of the nanofibers are expanded.