The invention discloses an endoscopic surgery instrument outer sheath based on a soft continuum mechanism, and belongs to the technical field of minimally invasive surgery instruments. The endoscopic surgery instrument outer sheath comprises a plurality of flexible units, each soft unit comprises a soft air with the high ratio of expansion and contraction, a rigid connecting piece, a steel wire soft shaft, a nickel-titanium wire, a disc, a surgical instrument channel, a pretightening force support, a pretightening force air cylinder and a fastening screw. Air pressure in the air bags is controlled to change the lengths of the air bags, and space bending of the units is achieved. Under any bent state, the pretightening force air cylinders apply pretightening force to the three nickel-titanium wires, the bending rigidity of the units is controlled, the effect of changing the rigidity is achieved, and the function of passive protection is achieved. The endoscopic surgery instrument outer sheath is used as an outer sheath unit of an endoscopic surgery instrument entering a human body, the outer sheath can be bent along with the cavity of the human body, the tissue of an inner wall is prevented from being oppressed, the rigidity is changed after the endoscopic surgery instrument outer sheath reaches the position of a disease, mechanical support is provided for the surgical instrument, the tissue can be prevented from being over-pressed to be hurt through the passive protection characteristic of the outer sheath when the surgical operation is carried out, and therefore the safety and effectiveness of surgery are improved.