Self-supported gel polymer electrolyte (GPE) was prepared based on copolymer, poly(methyl methacrylate–acrylonitrile–vinyl acetate) (P(MMA–AN–VAc)). The copolymer P(MMA–AN–VAc) was synthesized by emulsion polymerization and the copolymer membrane was prepared through phase inversion. The structure and the performance of the copolymer, the membrane and the GPE were characterized by FTIR, NMR, SEM, XRD, DSC/TG, LSV, CA, and EIS. It is found that the copolymer was formed through the breaking of double bond CC in each monomer. The membrane has low crystallinity and has low glass transition temperature, 39.1°C, its thermal stability is as high as 310°C, and its mechanical strength is improved compared with P(MMA–AN). The GPE is electrochemically stable up to 5.6V (vs. Li/Li+) and its conductivity is 3.48×10−3 Scm−1 at ambient temperature. The lithium ion transference number in the GPE is 0.51 and the conductivity model of the GPE is found to obey the Vogel–Tamman–Fulcher (VTF) equation.