Eucommia ulmoide gum (EUG) is a biomass-based polymer, and has a number of double bonds on the backbone, which can be post-functionalized. 1,2,4-Triazoline-3,5-dione derivative was synthesized and used to modify EUG by Alder-ene reaction, yielding a viscous modified EUG polyelectrolyte bearing 1,2,3-triazolium and polyethylene glycol pendants, which exhibited a relatively high ionic conductivity of 1.3 x 10(-5) S cm(-1) at 30 degrees C. The viscous EUG polyelectrolyte was further crosslinked by bis(1,2,4-triazoline-3,5-dione), and transformed to a solid state with a tensile strength of 2.0 MPa and an elongation at break of 150%. The crosslinked flexible EUG polyelectrolyte can take up ionic liquids with good compatibility to form the ion gel, and demonstrated an enhanced ionic conductivity of 1.4 x 10(-4) S cm(-1) at 30 degrees C when containing 50 wt% of ionic liquid, while maintaining good mechanical properties, which will have a potential application in the field of dielectric elastomer and flexible electronic devices.