The present invention provides a method for preparing a carbon nanofiber nonwoven, a carbon nanofiber nonwoven and a carbon nanofiber. The method for producing a carbon nanofiber nonwoven fabric enables the preparation of a carbon nanofiber nonwoven fabric capable of producing an electric double layer capacitor having a larger capacity than ever before. The preparation method comprises the following steps of preparing an incompatible polymer solution to prepare an incompatible polymer solution under the condition of forming a continuous phase with a phase comprising PAN to form a non-continuous phase and a PMMA-containing phase; a composite nanofiber nonwoven Cloth preparation process of producing a composite nanofiber nonwoven fabric composed of an aggregate of composite nanofibers from an incompatible polymer solution by an electrospinning method on an inner edge of a nanofiber shell made of PMMA A plurality of nanofiber cores made of PAN are arranged in a plurality of rows extending in a longitudinal direction of the nanofiber housing. A carbon nanofiber nonwoven fabric is subjected to a thermal treatment for carbonizing the PAN to carbonize the PAN while thermally decomposing the PMMA.