The invention provides a preparation method of a biomass charcoal/graphene flexible composite film. The nitrogen-rich biomass charcoal/graphene flexible composite film is prepared by the blended spinning solution containing the silk fibroin, the graphene oxide and the activator through the electrospinning technology and high temperature one-step carbonization activation. The material has the characteristics of being nitrogen-rich, porous and flexible. According to the method, the silk fibroin is rich in amino acids and can react with the graphene oxide in a cross-linking way through heating and stirring so that the graphene oxide is enabled to be aminated, the graphene oxide can be further reduced in the carbonization process, and the nitrogen atom part of the biomass residues in the composite material. The mass ratio of the biomass charcoal to the graphene in the obtained biomass charcoal/graphene flexible composite film is (0.5-9):1, the specific area is 1850-2740m2/g and the mitogencontent is 3.6-9.4%. The composite film is applied to the electrode material of the supercapacitor and the sodium ion battery so as to show high specific capacity, long cycle life and excellent rateperformance.