An inflatable assistive surgical instrument for transanal minimal invasive surgery, having a sheath that expands after inflation, is disclosed. The inflated sheath is cylindrical-shaped, and a notch is formed on the wall of the cylindrical sheath to expose the tissue of the colon wall for surgical operation. The inflatable sheath has an inflation nozzle for connecting with an inflating tube. The inflatable sheath disclosed in the present invention can fully achieve the expected effects and requirements and shows an extraordinary efficiency of the surgical operation, in comparison with the prior arts where the present invention is not adopted. The present invention also improves the quality and the success rate of surgical operation, and is highly appraised by the colorectal surgeons reduces the cost for single use.