The invention discloses a preparation method of a tobacco stem-based composite hydrogel. The preparation method of the tobacco stem-based composite hydrogel comprises the following steps of (1) mixingpretreated tobacco stems, an acrylic acid and sodium acrylate neutralizing solution, an initiator and a cross-linking agent, irradiating the mixed solution through an ultraviolet lamp for 3 to 25 minat the room temperature to obtain a composite hydrogel, wherein the power of the ultraviolet lamp is 50 to 500 W, the distance between the ultraviolet lamp and the mixed solution is 15 to 50 cm, theneutralizing degree of the acrylic acid and sodium acrylate neutralizing solution is 75 percent to 85 percent, the mass of the initiator is 0.85 percent to 1.25 percent of the mass of the acrylic acidand sodium acrylate neutralizing solution, and the mass of the cross-linking agent is 0.05 to 0.08 percent of the mass of the acrylic acid and sodium acrylate neutralizing solution; (2) soaking the composite hydrogel in methyl alcohol or ethyl alcohol, standing for 12 to 24 h, removing an unreacted monomer and an incompletely reacted oligomer, and drying, wherein the drying temperature is 50 to 100 DEG C, and the drying time is 4 to 12 h. According to the preparation method of the tobacco stem-based composite hydrogel provided by the invention, the tobacco stems with a relatively low utilization value can be converted into the composite hydrogel with high added value.