In order to solve the problem of high expansion coefficient and poor thermal stabilityof the flexible organic solar cellsubstrates, this paper proposed a method that using cellulose nanofibrils to reinforce epoxy resin as the flexible substrate of organic solar cells. In this paper, the nanocellulose with highly transparent and outstanding thermal stability properties were took to improve the epoxy’s thermal stability that possesses high thermal expansion coefficient, therefor a new nanocomposite substrate material is achieved by preparing a transparent film of nanocellulose impregnated with epoxy resin. The results shown the thermal expansion coefficient of composite film decreases from 46×10-6/K to 19×10-6/K comparing with the pure epoxy resin, and the new substrate also has outstanding light transmittance(89%), smooth surface (roughness 2.15nm), good mechanical properties (Young'smodulus 2.3GPa, tensile strength 59MPa). According to this results, a poly(3,4-ethylenedioxythiophene):polystyrene sulfonate conductive electrode was deposited on the nanocomposite substrate and the surface morphology and conductivity were performed between -15°C to 70°C. The results proved that electrode materials deposited on the substrate hadn’t any crack when the temperature changed, and the conductivity remained stable (830-875S/cm).