The invention belongs to the field of supercapacitors and discloses a flexible and solid supercapacitor, a preparation method and application thereof. The supercapacitor comprises a positive electrode, a negative electrode and an electrolyte interposed therebetween, wherein the electrolyte is double-network lignin hydrogel, and both the positive and negative electrodes are conductive electrodes. The double-network lignin hydrogel as the electrolyte in the present invention not only has high mechanical strength property, but also has a simple preparation method, low raw material cost, renewability, green environmental protection, and high ionic conductivity; when the hydrogel is used as the electrolyte to assemble the supercapacitor, additional soaking of an ionic solution is not required and additional use of a diaphragm is not needed, thereby effectively reducing the thickness and weight of the supercapacitor. The obtained capacitor specific capacitance value is higher than the current hydrogel electrolyte type flexible supercapacitor specific capacitance value. In addition, the supercapacitor has very good charging and discharging cycle stability, compression resistance and bending resistance.