A composite membrane, a biosensor, and preparation methods thereof are disclosed. In the preparation process, a moldable and plastic material is used as a carrier; after a dispersion liquid of a nanocarbon material wets the carrier, by means of multiple stretching and folding processes, the nanocarbon material is uniformly distributed and arranged regularly inside the entire plastic material in a stretching strain direction. The prepared composite membrane is elastic, attachable, and cost-efficient, and may detect strains as high as 530%. The composite membrane further has high sensitivity and high durability, and may be effectively applied to a biosensor for monitoring motion and humidity.