A preparation method of physically-crosslinked chitosan-based hydrogel with efficient pH sensitivity and high mechanical strength, wherein the hydrogel is shown as the formula (I), belongs to the technical field of high-molecular functional modification synthesis of natural materials. The method comprises: taking hydroxy groups located on the surface of chitosan shown as the formula (III) as grafting sites; allowing potassium persulphate-sodium hyposulfite which is taken as a chemical initiator to be heated and decomposed to generate free radicals; on the active sites formed on chitosan, further triggering a part of monomers AAC shown as the formula (IV) and monomers DMAEMA shown as the formula (IV) to be grafted to main chains of chitosan through polymerization, while obtaining copolymermolecular chains of the two monomers, wherein the copolymer molecular chains are intermixed among macromolecular chains of chitosan grafted copolymer to form semi-interpenetrating network, thereby obtaining the hydrogel shown as the formula (I), and performing freeze drying to the hydrogel to obtain a solid product. The reaction is easy to operate, and the condition is mild. The physically-crosslinked hydrogel obtained from the preparation method is efficient in pH sensitivity, high in mechanical strength, good in biological compatibility and large in swelling rate, and has bright applicationprospects with the respect to macro-molecule solute separation, drug release systems, disease diagnosis and daily monitoring in the medical and medicine fields.