The invention relates to the technical field of strain sensors, and in particular, relates to a preparation method of a high-sensitivity piezoresistive strain sensor. The method comprises the steps: adding distilled water into polyvinyl alcohol, firstly swelling, then stirring and heating until the polyvinyl alcohol is completely dissolved to prepare polyvinyl alcohol gel, and cooling to room temperature for later use; dropwise adding a glutaraldehyde solution into the polyvinyl alcohol gel, uniformly stirring, and standing at room temperature to obtain polyvinyl alcohol/glutaraldehyde gel; dropwise adding a prepared ferric trichloride aqueous solution into the polyvinyl alcohol/glutaraldehyde gel, uniformly stirring, immediately transferring into a polytetrafluoroethylene mold, freezing,and freeze-drying to obtain a ferric trichloride/polyvinyl alcohol composite membrane; and adding a pyrrole monomer into an aqueous solution of acetonitrile, uniformly stirring to form a monomer solution, soaking the ferric trichloride/polyvinyl alcohol composite membrane into the monomer solution, reacting at room temperature, and washing with methanol to obtain a polypyrrole/polyvinyl alcohol composite membrane, namely the high-sensitivity piezoresistive strain sensor.