The continuum over tube applied to natural orifice transluminal endoscopic surgery (NOTES) should be flexible during insertion and provide sufficient stiffness upon arrival at the lesion. In order to satisfy these two properties, a variable stiffness continuum over tube was proposed. The over tube realizes stiffness variation through the heating and cooling of filling materials. Polyethylene glycol, cocoa butter, and gallium metal are selected as filling materials. Several experiments were designed to test its rigid and response characteristic, and suitable material was selected. The risk management test was carried out, and the frequency and risk level of the risk point were recorded. According to experimental data, the polyethylene glycol and gallium show better variable stiffness performance, with the 20 times stiffness variation range, and polyethylene glycol was selected as suitable materials for its low cost and nontoxic characteristic. The average heating time and cooling time of polyethylene glycol-filled over tube are 52.3 s and 36 s, respectively. Moreover, three kinds of high-frequency risk points including smoke, thread ejection, and uneven distribution of the material were found, and corresponding design improvement and use principle are proposed. The proposed continuum over tube can satisfy the requirement of colorectal NOTES for flexibility and stiffness of instruments, and the safety of the instrument can be improved by controlling high-frequency risk points with relevant methods.