The invention discloses a method which is used for preparing 2-pentene by separating petroleum C5 fraction and takes raffinate C5 obtained after separating diolefin and isoamylene by extracting as raw materials. The process comprises that the raw materials are extracted and distilled, and the extractant is N and N-dimethylformamide, and the feeding weight ratio of the raw materials and the extractant is 1:5 to 1:12, C5 saturated hydrocarbon materials are discharged from a column top and the materials richly containing 2-pentene and the extractant are obtained in a column bottom; the materials in the column bottom obtained in the process 1 are distilled so as to recycle the extractant, and the materials richly containing 2-pentene are obtained in the column top, and the extractant is obtained in the column bottom, and the extractant can be recycled; the materials in the column top obtained in the process 2 are distilled and separated so as to remove light components, and the materials with light components are discharged from the column top, and the crude product of 2-pentene is obtained in the column bottom; and the crude product of 2-pentene obtained in the process 3 is distilled and refined so as to remove heavy components, materials in the column top are taken and a refined product of 2-pentene is obtained. By processing at a much deeper degree, the method of the invention solves a technical problem that 2-pentene in C5 fraction can not be utilized by the prior art.