The invention relates to a biopolymer chitosan composite polymer electrolyte and a preparation method thereof and a battery. The preparation method specifically comprises the following steps: S1, chitosan (CS) is added into an acetic acid solution to prepare a chitosan suspension, and stirring is conducted for 24h; S2, polyethylene oxide (PEO) and a lithium salt are added into an acetonitrile solution and stirring is conducted for 4-6h to form a uniform suspension; S3, the chitosan suspension obtained in S1 and the suspension obtained in the S2 are mixed, the mass ratio of chitosan (CS) to thelithium salt is 1: (1-1.2), and stirring is conducted for 6-12h to form a uniform mixed solution; and S4, the mixed solution prepared in S3 is poured into a polytetrafluoroethylene mold, and the mixed solution is dried at room temperature to finally obtain a polymer electrolyte membrane. The polymer electrolyte membrane prepared by the method disclosed by the invention has good mechanical properties and electrochemical properties, is simple in preparation process and low in cost, and is expected to replace a diaphragm and a traditional electrolyte to be prepared into an all-solid-state or semi-solid-state battery.