The invention discloses a preparation method of a silicon-modified multi-wall carbon nanotube flexible composite electrode. The method comprises the following steps: adding multi-walled carbon nanotubes into trimethylolaminomethane solution, applying ultrasonic waves, adding dopamine hydrochloride, stirring, centrifuging to obtain polydopamine-coated multi-walled carbon nanotubes, adding the product into a solution composed of NaOH and n-dodecyl mercaptan, and standing for 14-16 hour later, and obtaining PDCNT after centrifugal washing and drying; mixing PDCNT N-hydroxysuccinimide and ethanolare mixed, and then adding to silicone oil paper dropwise after ultrasonic treatment, coating with a scraper, and drying to obtain a PDCNT flexible film; and preparing slurry by dissolving nano Si particles, Super P Li and polyacrylic acid in ethanol, fixing the prepared slurry-coated PDCNT flexible film on a glass plate with the scraper, drying, rolling, cutting into pieces and obtaining the flexible composite electrode. The flexible electrode prepared by the method has good flexibility and bending resistance.